PLANT BREEDING
Principles and Methods
B.D. Singh, Professor
Department of Genetics and Plant Breeding
Institute of Agricultural Sciences
Bana^s Hindu University
Varanasi
KftbYANI PUBbISHBRS
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Nlkbli
Nisfelth and Pushpa
KALYANI PUBLISHERS
H.O . : 1/1, Rajinder Nagar, Ludhiaoa-J41 008.
j 9.0, ; 4863/2B, Bharat Ram Road,
24, Dsryagaaj, New Delhi*! 10 002.
© 1983, B.D. SINGH
Fourth Edition 1990
Reprinted 1990
ISBN 81*7096-308-7
PRINTED IN INDIA
At Kalyani Printings, B-15, Sector 8, NOIDA
and published by Mrs. Usha Raj Kumar for
Kalyani Publishers, New Delhi- 1 10 002
Foreword
The contributions made by Indian plant breeders during the
last three decades are well known to the outside world. It may be
favourably compared with achievements of plant breeders in any
other country. But the work of Indian plant breeders has not been
compiled at one place so that students of plant breeding in India
may have an easy access to it. Indian students hardly find a suitable
textbook on plant breeding except those written by Allard ; Elliot ;
Briggs and Knowels ; Hayes, Immer and Smith ; and Simmonds,
which were never written for Indian conditions. Only two text books
relate particularly to the Indian context : these are ‘Breeding of
Asian Field Crops’ by Poehlman and Borthakur and ‘Elementary
Principles of Plant Breeding’ By Chaudhari. The first work gives
an excellent cropwise treatment of breeding methods used and salient
achievements but it does not deal with the principles and the breed-
ing methods to an appreciable extent. The book by Chaudhary
does not touch on several relevant topics, e.g., population improve-
ment in cross-pollinated crops, polyploidy in plant breeding and
distant hybridization in crop improvement.
The present book, ‘Plant Breeding : Principles and Methods’
by Dr. B.D. Singh, Professor in the Department of Genetics and
Plant Breeding of this Institute is a welcome effort. He has attempted
to write a text book in the real sense with adequate emphasis on
principles as well as on methods of plant breeding. The beauty of
this book is that the examples are taken from the Indian conditions.
He has particularly devoted a chapter to ‘Organisations for Crop
Improvement in India’ which also outlines the history of agricultural
research and development in the country. In addition, a chapter on
‘In Vitro Methods’ highlighting the possibilities offered by this novel
technique adds tremendous value to the book. Students of plant
breeding in India will definitely find this book a real help in getting
the background of the work done in the past and the future projec-
tions of the subject. As such, this book bridges the gap of not hav-
ing a suitable text book in plant breeding for Indian conditions. The
language of the entire book is such a simple and easy one that even
the complicated topics of the subject will be understood well by the
readers. The figures and drawings used in the book make the text
more easily understandable.
<iv>
I personally congratulate Dr. B.D. Singh for his maiden at-
tempt at writing such a valuable textbook. At the same time, I am
confident that this book will be widely accepted in the universities,
colleges and research institutions engaged in evolution of crop
varieties.
1983 (MAHATIM SINGH)
Director
Institute of Agricultural Sciences
Banaras Hindu University
Varanasi
Preface
( Fourth Edition)
I feel exhilarated in writing the preface for the fourth edition
■of Plant Breeding : Principles and Methods. Many additions have
•been made in this edition, e.g., a new chapter 'Effectiveness of the
breeding methods for seif-pollinated crops’, a thorough revision \of
the chapter ‘Quality seed : classes, production practices and main-
tenance’, inclusion of ‘doubled haploid technique in the chapter*
‘other approaches to breeding of self-pollinated crops’ etc. These
additions, it is hoped, will enhance the usefulness of the book.
In the third edition of the book,, two new chapters, viz
‘Breeding for insect resistance’ and ‘Release of new varieties’, were
included. These chapters contained many errors, which I have
attempted to remove in this edition. The details of the procedures
for the testing of new strains, more particularly the zoning patterns,
are still in a flux, and are expected to remain so for some time in the
future,
I have benefited from discussions with many students and
teacher colleagues who provided constructive and well-intended
criticisms as well as suggestions for new inclusions. I am grateful
to Late Dr. G.S. Sharma, Varanasi, for providing the brasic infor-
mation for the chapter ‘Release of new varieties’, and to Dr. P. Singh,
Nagpur, for having prepared the initial draft for the chapters
‘Biometrical techniques in plant breeding’ and ‘Breeding for insect
resistance’, and for several other contributions and suggestions for
improvement. Dr. Haribar Ram* Pantnagar, was kind to go through
and comment on ‘Release of new varieties’, while the comments of
Dr. D.P. Singh, Faizabad, provided the basis for a thorough revision
of the draft on ‘Effectiveness of the breeding methods for self-polli-
nated crops’. Thanks are also due to Dr. B.S. Manake, Dhule,
Dr. R.K. Raghuvanshi, Jaipur, Dr. L. Krishnamurthy, ICRISAT,
Hyderabad, Dr. G.S. Gagtap, Khargone, M.P. and Mr. A.K. Singh
for their valuable suggestions,
I am aware that many further improvements are due in the
book in view of the very rapid accumulation of new findings. It is
4his challenge of a book from going obsolete which provides the
ultimate thrill to an author,
1990 BJX Singh
Varanasi
Preface
(1st Edition)
The present text book has been prepared for B.Sc. (Ag.)
students of Indian universities* An attempt has been made to pre-
sent the principles and methods of plant breeding in a simple lan-
guage' and to use suitable examples from the Indian context. Tables
and charts have been used, where necessary, to make the text clearer,
and it is hoped that this would be of some help to the students.
There is a certain amount of repetition throughout: the book, which
is intentional in the hope that this would act as a reinforcement for
the memory.
A text book has to oscillate between two opposite and denud-
ing necessities. The first necessity is to keep the text easily under-
standable, which necessitates the use of a simple language and parti-
cularly, -elaborate exposition. The second necessity relates to
providing as much uptodate information as possible within the
limitations of the space in the text ; this makes it essential to be
concise and to use a technical language which is relatively more
difficult to comprehend for a beginner. . An emphasis’ on the first
need is likely to make the text too elementary, while an emphasis on
the second would adversely affect its comprehensibility. The success
of an author, therefore, would primarily depend on how well he has
been able to achieve an acceptable balance between the two opposing
demands. ' The point of this balance, by its nature, would be highly
subjective, but the norm would be the majority view. Whether this
balance has been achieved in this text would, therefore, be known
only after the reactions of the readers are available.
A text book can hardly claim originality either in content or
in presentation. The present text book is no exception and it draws
heavily from earlier texts- on plant breeding, notably. Principles of
Plant Breeding by R.W. Allard, Principles of Crop Improvement by
N.W. Simmonds, Breeding Asian Field Crops by J.M. Poehlman and
D f Borthakur, Methods of Plant Breeding by H.K. Hayes, F.R.
Immer and D.C. Smith and Plant Breeding and Cytogenetics by
F.C Elliot, and from other published works, notably, A History of
the Indian Council of Agricultural Research by M.S. Randhawa.
Fifty Years of Agricultural Research and Education by ICAR and
Vistas in Crop Yields by ICAR.
in addition, ! have received help from my colleagues, Dr.
R.M. Singh, Dr. A.K. Richharia, Dr. V.P. Singh and Dr. U.P.
Singh who have contributed in one way or the other in the develop- . •
wAnt nf this text. I wish to express my appreciation for Dr. Mahatim
Singh, Director, Institute of Agricultural Sciences, Banaras Hindu
University, for his keen interest in the project and for some valuable
suggestions regarding the manuscript. My students Sri R.P. Singh,
Parshotam Singh, H.K. Jaiswal, Vinod Tiwari and P .S. Bhatnagar
have contributed in collection of material and correction of the
typescript.
My colleagues, students and the literature have contributed to
whatever usefulness and quality is present in the text, but the errors,
omissions and deficiencies are my own copyright and no one else
should be credited for them.
However, all this would not have been possible without my
long association with Professor R.B. Singh, Plant Production and
Protection Officer, F.A.0., Bangkok, who instilled in me the habit
of spending long hours with the work at hand.
In the end, I must express my appreciation for my wife Pushpa
and my sons Pintu and Cheenu who have been affectionate, under-
standing and patient during the long hours*I spent on the manuscript,
which often were very trying indeed.
B.D. Singh
Contents
Chapter
Introduction to Plant Breeding
\JKstory of plant breeding. Nature of plant breeding,
What should a plant breeder know ?, Objectives of
plant breeding. Activities in plant breeding. Some
important achievements, Future prospects, Summary.
Domestication, Plant Introduction and
Acclimatisation
Domestication, Patterns of evolution in crop plants.
Centres of origin, Plant introduction, History of
plant introduction, Plant introduction agencies in
India, Procedure of plant introduction, Germplasm
collections. Exploration, Purpose of plant introduc-
tion, Some important achievements of plant intro-
duction, Merits of plant introduction, Demerits of
plant introduction. Summary.
Modes of Reproduction and Pollination Control
Modes of reproduction. Asexual reproduction,
Sexual reproduction, Anthesis, Modes of pollina-
tion, Self-pollination, Cross-pollination, Determina-
tion of mode of reproduction in a species,
Determination of the amount of ^gross-pollination
in a species, Relevance of mode of reproduction to
plant breeding, ^Mechanisms of pollination control
in crop plants, Self-incompatibility, Male sterility.
Summary.
Qualitative and Quantitative Characters
Inheritance of qualitative characters, Pleiotropy,
Penetrance and expressivity, Threshold characters.
Modifying genes, Gene interaction, Linkage, Quan-
titative characters, The multiple factor hypothesis,
Polygenic inheritance and continuous variation.
Role of environment in quantitative inheritance.
Components of genetic variance, Estimation of
components of genetic variance, Heritability, Sum-
mary.
Chapter
5, Biometrical techniques in Plant Breeding
Assessment of variability, simple measures of varia-
bility, D 3 statistic, metroglyph analysis ; Aids to
selection, Correlation coefficient analysis, path
analysis, discriminant function ; Aids to choice ol
parents and breeding procedures, diailel cross analy-
sis, partial diallel analysis, tnallel analysis, quad-
riallel analysis, linex tester analysis, generations
mean analysis, biparcnta! cross analysis ; Study of
varietal adaptation ; Summary.
<f. Selection in Self-Pollinated Crops
History of selection, The progeny test, The pureline
theory, Effects of self-pollination on genotype,
Origin of variation in purelines, Genetic advance
under selection, Expected genetic advance in segre-
gating populations. Summary.
7. Hybridization : Techniques and Consequences
History of hybridization, Objectives of hybridiza-
tion. Types of hybridization, The hybridization
programme, The procedure of hybridization, Raising
the Fi generation, Selfing, Consequences of hybridi-
zation, Summary.
8. Genetic Composition of Cross-Pollinated Populations
The Hardy- Weinberg law. Factors disturbing the
equilibrium in populations. Systems of mating.
Random mating, Genetic assortative mating, Gene-
tic disassortative mating. Phenotypic assortative
mating, Phenotypic disassortative mating, Summary.
9. Selection in Cross-Pollinated Crops
Rapid gain followed by show response. Continued
slow progress for -a long period, Slow response for
a short period, Lack of response to selection, Rapid
gain— Plateau— Rapid gain response, Summary.
10. Heterosis and Inbreeding Depression
Inbreeding depression, Effects of Inbreeding,
Degrees of inbreeding depression, Homozygous
and heterozygous balance, Heterosis, Historical,
Heterosis in cross- and ^gglftpollinated species, Gene-
tic basis of heterosis and inhrp^dins denression.
Pages
100—125
126—137
138—154
155—166
167—176
177-193
Chapter
Dominance hypothesis, Over-dominance hypothesis.
Physiological basis of heterosis, Commercial appli-
cations, Summary.
11. Mass Selection
Applications ' of mass selection, The procedure of ■
mass selection, Merits of mass selection. Demerits
of mass selection, Achievements, Summary.
12. Poreline Selection ‘
Characteristics of purelines, Uses of purelines,
History of pureline selection, Applications of
purelioe selection, A general procedure for poreline
selection. Advantages of poreline selection, Dis-
advantages of pureline selection, Comparison
between pureline and mass selection, Achievements,
Summary.
13. Pedigree Method
Pedigree record. Maintenance of pedigree record,
Applications of pedigree method. The procedure
, of pedigree method. Basis of selection, . Early
generation tests, Off-season crops, Merits of
pedigree method, Demerits of pedigree method,
■ Achievements, Summary.
14 Bulk Method
Applications,, The procedure of bulk method.
Artificial selection - during, the bulking period.
Duration of, bulking, Natural selection during
bulking period, A modification of the -bulk method,
Single-Seed-Descent Method, Merits df bulk method,
Demerits of bulk method, Achievements, Com-
parison between, bulk -and pedigree methods,
Summary.
IS. Backcross Method
Requirements of a backcross programme, Applica-
tions of backcross method, Genetic consequences of
-Repeated backcrossing, Selection of parents, The
.. procedure ■ of backcross method. Number of plants,
necessary in backcross generations. Selection for
The ’character : being 'transferred, Number ' of back-
crosses to be made, Transfer of Leaf Rust Resistance
■ , to Malviya 12 Wheat, Transfer of quantitative
characters, Transfer of two or more characters to
194 — 200
201—209
210 — 223
Chapter
16 .
17.
a single recurrent leaf, Modifications of backcross
method Application to cross-pollinated crops.
Merits of backcross method. Demerits of backcross
method. Achievements, Comparison between back-
cross and pedigree methods. Summary.
Other Approaches to Breeding of Self-Poilinated
Crops
Multiline varieties. Merits of multiline varieties
Demerits of multiline varieties, Achievements, Popu-
lation approach to Breeding of Self-pollinated
crops, Merits of population approach, Demerits of
population approach, Hybrid varieties, Summary.
Effectiveness of the Breeding Methods for
Self-Pollinated Crops
Pedigree method Single seed descent, Pedigree vj.
bSD, Doubled haploid technique. Bulk method
Bulk vs. PS and SSD, Conclusion. ’
255
269
300-
18. Population Improvement
Objectives of selection, Mass selection. Merits of mass
selection, Demerits of mass selection, Effectiveness
nf mac<f selection, Progeny selection, Applications
of mass selection and progeny selections, Achieve-
ments with mass and progeny selections. Recurrent
selection. Simple recurrent selection. Recurrent
selection for GCA, Recurrent selection for SCA
Reciprocal recurrent selection, Comparison between
different recurrent selection schemes. General con-
Summary/' 001 i"
19. Hybrid and Synthetic Varieties 328 .
Definitions, History of hybrid varieties. Operations
m production of hybrid varieties, Development of
rnbreds, Evaluation of inbreds, Production of
hybrid seed. Improving the characters of inbred
lines, Achievements through hybrid varieties. Syn-
thetic varieties, Operations in producing a synthetic
variety Eyaiuation of lines for GCA, Production
of synthetic variety, Multilocation of synthetic
varieties, Merits of synthetic varieties. Demerits of
^, nthel * c varieties, Factors determining performance
of synthetic varieties. Maintenance of synthetic
Summary T?StS ^ combinin8 abilit y. Achievements,
Chapter
20. Clonal Selection and Hybridization
Characteristics of asexually propagated crops,
Clone, Genetic variation within a clone, Compari-
sion among clones, Purelines and inbreds, Clonal
degeneration. Methods of improvement of asexually
propagated crops. Clonal selection, Hybridization,
Inter-specific hybridization in improvement of clonal
crops, Problems in breeding of asexually propagated
crops, Achievements, Summary.
21. Breeding for Disease Resistance
Losses due to diseases, History of breeding for
disease resistance, Mechanisms for generation of
variability in pathogens, Physiological races and
pathotypes. Genetics of pathogenicity. Disease
development. Disease escape, Disease resistance.
Vertical and horizontal resistance. Mechanisms of
disease resistance. Genetic basis of disease resis-
tance, Gene-for-gene relationship, Sources of
disease resistance. Methods of breeding for disease
resistance. Testing for disease resistance. Disease
epidemics. Causes of epidemics. Prevention of
epidemics, Summary.
22. Breeding for Insect Resistance ;
Losses due to Insects, Genetic Variability in Insect
Pests, Mechanism of Insect Resistance, Nature of
Resistance, r Genetics of Insect Resistance, Sources
of Resistance, Breeding Methods for Insect Resis-
tance, Screening Techniques, Durability of Resis-
tance, Prevention of Insect outbreaks, Problems in
Breeding for Insect Resistance, Achievements,
Breeding for Resistance ’ to Parasitic Weeeds,
Summary.
23. Mutations in Crop Improvement 4
Historical Account, Spontaneous and induced
mutations. Effects of mutation. Mutagens, Mecha-
nism of action of radiations. Mechanism of action
of chemical mutagens. Methods of mutation breed-
ing, Recurrent irradiation. Gamma-garden, Directed
mutagenesis, Applications of mutation breeding.
Limitations of mutation breeding, Achievements,
Summary.
Pages
356—368
369-396
397-414
(xiv)
■ Chapter
24. Polyploidy in Plant Breeding
28 .
Pages
442-474
Types of changes in chromosome number, History
of heteroploidy, Aneuploidy, Applications of alien-
ploidy io crop improvement, Aulopolyploidy,
Origin and production of doubled chromosome
numbers, Morphological and cytological features
of autopolvploids, Segregation in autopolyploids,
Role of autopolyploidy in evolution, Applications
of autopolyploidy in crop improvement. Limitations
of autopolyploidy. Allopolyploidy, Origin and
production of allopolyploids Role of allopolyploidy
in evolution. Applications of allopolyploidy in
crop improvement, Limitations of allopolyploidy.
Summary.
25. Distant Hybridization in Plant Breeding
475— 49S
History, Barriers to production to distant hybrids,
Techniques for production to distant hybrids.
Sterility in distant hybrids. Consequences of segre-
gation in distant hybrids, Applications in crop
improvement, . Limitations, Achievements, Sum-
mary.
In Vitro Techniques in Plant Breeding
496—509
History, The in vitro technique. Classification,
Embryo culture, Meristem- culture. Anther and
pollen cultures. Tissue and Cell cultures, Achieve-
ments and future prospects, Summary.
27. Release of New Varieties
510—525
Evaluation, Identification of Entries for Release,
Multiplication, Summary.
Improved Seed : Classes, Production Practices and
Maintenance
526—564
Classes of improved seed, Requirements of certified
seed, Production and' distribution of improved
seed, Seed production and processing. The Indian
Seeds Act (1966), Seed certification. Field insection.
Seed tests, Maintenance of. improved seed. Seed
production organisations, Certified seed production
in : Self-pollinated crops, Hybrid maize, Hybrid
jowar. Hybrid bajra, Potato, Summary.
Cliapt* Fages
29. Organisations for Crop Improvement in India 565—590
History of agricultural research in India, Establish-
ment of agricultural departments and colleges
Establishment of ICAR, The commodity commit-
tees, PIRRCOM centres, Initiation of all India
coordinated research projects, Reorganisation of
ICAR, Development of agricultural universities,
Organisations for crop improvement, Organisation
and functions of ICAR, Central institutes for crop
improvement. Agricultural universities, AH India
co-ordinated crop improvement projects, Ad hoc
research projects. Summary.
30. International Institutes for Crop Improvement 591—598
The international institutes. Functions of inter-
national institutes. Some contributions of inter-
national institutes, Summary.
Glossary "V 599—620
Index 621
CHAPTER 1
Introduction' to Plant Breeding
.The foodgr&ie production is India has increased from 54.92
milliaa tonnes (from 99.28 million hectares) in 1949-50 to 150.47
million tonnes (from 127.06 million 'hectares) in 1985-86. This repre-
sses!® an increase of about 274 per cent during a period . of 36
hs a result, the nation has become almost self-sufficient in
foodgraisa. But the population in Isdia is growing at an alarming
gate of 2.5 per cent per year. This makes it necessary that the food-
psiE production should also increase at least at the same rate or
ev®e at a faster rate in order to improve the nutritional status of the
masses.
This large increase in fcodgratn production has resulted from
an increase is act cropped area, (127.06 million hectares in 1985-86
as against 99.28 million hectares in 1949-50), increased quantum
rid better management of inputs, such as fertilizer, irrigation water,
plant protection and cultural practices, and from improved crop
varieties (Fig. 1.1). It is doubtful that the net cropped area
can be increased indefinitely, but there is stiii some scope through
doable and multiple cropping. Improved input management practices
5 HIGH YIELDING
| VARIETIES
«
BETTER INPUTS
AND MANAGEMENT
INCREASE IN
NET CROPPED
AREA
INCREASED FOOD l:
PRODUCTION !
• j ;
1% *J4» • JRs|ptas»4Wlsit>«»g food production i& India. i;-;
2 Plan! Breeding : Principles and Methods
are yet to be fully exploited, and vast tracts of cultivated
lands are very poorly managed. In future, agricultural produc-
tion is most likely to Increase from providing better environ-
ment ^ through management and from crop-' varieties capable of fully
exploiting the environment provided. Better environment alone cannot
lead to better yields from inferior varieties beyond a certain limit.
The limit of yield is set by the genetic makeup of the variety. Im-
proving the environment beyond a certain limit for any variety may
adversely affect its performance. This point is illustrated by the tall
and dwarf varieties of wheat. Tall wheat varieties respond to
nitrogen application upto 60 kg/ha, but higher doses of nitrogen
reduce yields primarily due to severe lodging. Dwarf wheat
varieties, on the other hand, give increased yields upto, or even
beyond, 120 kg nitrogen per hectare. It is believed that the genetic
makeup of the crop plants is such that it would permit considerable,
perhaps eadless, changes and improvements. Further, the changes
brought about in the present varieties probably represent only a
small portion of the possibilities. Thus continuous favourable
changes in the genotype of crop varieties are a must for increasing
yields from crop plants. *
Plant breeding- deals with this aspect of crop production :
It consists of the principles and the methods required for favourably
changing the genetic constitution of crop plants. This usually produces
crop varieties more suited to human needs in one or more aspects
than the existing ones. v
HISTORY OF PLANT BREEDING
. It is reasonable to assume that plant breeding, howsoever
primitive, began when man first chose certain plants for cultivation.
The process of bringing a wild species under human management is
referred to as domestication. There is bound to be some selection
during domestication: This is likely to give rise to better types than
the wild ones. Thus domestication may be regarded as a method of
plant breeding. Domestication continues till today, and is likely to
continue for some time in future. This is particularly true in the case
of timber trees, medicinal plants, microbes and plants satisfying
some special requirements. A remarkable case of domestication in
recent years was that of Penicillium for penicillin and subsequently of
several other fungi /or various antibiotics. Transfers of specific genes
c.g„ for disease resistances, from wild species to cultivated ones mav
™ of those g Mra , ; . e „ „f a part „f lhe J
Daring the long period of prehistoric and historic cultivation
?t S“l S h “ d 1“ ite,y ac 5 d domestiStS “S:
It is likely that man also exerted some selection knowiturlv nr
unknowingly. Movement of man from one area to anotha bfoLht
about the movement of his cultivated plant s^cfes SaSSSSSS
Introduction to Plant Breeding
3
into an . area of new plant species or varieties from other parts of the
world is an integral pan of plant breeding today ; this aspect will be
covered in a greater detail in Chapter 2.
Babylonians and Assyrians pollinated date palm artificially as
early as 700 B.C. In 1717, Thomas Fairchild produced the first
artificial hybrid. Joseph Kolreuter, a German, made extensive
crosses in tobacco between 1760 and 1766. Knight (1759-1835)
was perhaps the first man to use artificial hybridization to develop
several new fruit varieties. Le Couteur and Shireff, around 1840,
used individual plant selection and progeny test to develop some
useful cereal varieties. Vilmorin (1856) further developed the
■progeny test and used this method successfully in improvement of
sugarbeets (Beta vulgaris). The individual plant selection method was
S developed in detail by Nilsson and his associates at SvaJof, Sweden
r ' around 1900. In 1903, Johannsen proposed the pureline theory that
1 , provided the genetic basis for individual plant selection.
The modern plant breeding methods have their basis , in the
genetic and cytogenetic principles. The science of genetics began
i with the rediscovery of Mendel’s paper in 1900, which was
«| originally published in 1866. Gregor Mendel’s laws of inheritance
provided the basis for the vast knowledge that has accumulated in
genetics. Numerous workers who determined the various modes of
inheritance have contributed to the development and understanding
of plant breeding. Realisation that the chromosomes are the carriers
of genes has led to the development of specialized plant breeding
methods for chromosome engineering. A noteworthy development
resulted from the studies of G.H. Shull on inbreeding in maize
(Zea mays). He found that inbreeding produced a considerable loss
of vigour. But when some of the weak inbred- lines were crossed, the
resulting hybrids were more vigorous than the original variety.-
These observations Jed to the production of hybrid varieties which
are common, in the case of maize, jo war (Sorghum bicolor), bajra
{Pennisetum americanum) and several other crops.
NATURE OF PLANT BREEDING
Science is the knowledge gathered through scientific methods.
The scientific method consists of observation, formulation of a
hypothesis, experimentation and conclusion either to accept or to
reject the hypothesis. Thus science is objective in approach, while
art is highly subjective. Whether plant breeding is an art or a science
is hardly a point of dispute anymore. In early days, man depended
on his skill in selecting better plants. His knowledge about the plants
was 'very- limited. He knew nothing about inheritance of . characters,'
■role of environment in producing them and the basis of variation in
various; plant, characteristics. • His methods of selection were designed
without an understanding of the principles of inheritance. Therefore,
4 Plant Breeding : Principles and Methods
plant breeding then was largely, often exclusively, an art. Bat the
present plant breeding methods are based on scientific principles of
plant sciences, particularly of genetics and cytogenetics. Thus, a
large part of the breeding work is purely science with very little art
involved. However, the selection of desirable plants even today is
largely an art since much depends upon the ability of the breeder to
identify superior plants by visual observation. The amount of seed
set in crosses also depends, to a large degree, on the skill of
person making the crosses. Thus plant breeding today is mostly a
science. A modern plant breeder has to know all be can about his
plants, and that involves knowledge of several related disciplines.
THE DISCIPLINES A BREEDER OUGHT TO KNOW .
To be successful, a plant breeder must know all -he can
about the plants he is working with. Thus he should have an under-
standing of the following : (1) botany, (2) genetics and cytogenetics,
(3) agronomy. (4) plant physiology, (5) plant pathology, (6)
entomology, (7) bacteriology,. (8) plant biochemistry, and (3)
statistics.
Botany. A plant breeder must have a clear understanding of the
morphology and reproduction of the plants he aims to improve. He
should also be familiar with the taxonomy of the plant.
Genetics and cytogenetics. The principles of genetics and cyto-
genetics provide the basis for plant breeding methods. Therefore, a
thorough knowledge of these subjects is essential for a rapid and
efficient improvement of a crop plant.
Agronomy. A good breeder is first a good agronomist. He
must be able to raise a good crop in order to select and evaluate his
material
Plant Physiology. Adaptation of a variety is determined by its
response to environmental factors like heat, cold, drought, salinity
etc. A knowledge of the physiological basis of these responses would
help the breeder in developing cold, drought or salinity tolerant
varieties. In addition, several physiological approaches to breeding
for higher yields are being developed.
Plant Pathology. Breeding for disease resistance is an important
objective of plant breeding. For an effective breeding for resistance, a
sound knowledge of plant diseases and their pathogens is
■ essential.
Entomology. Insect pests cause considerable damage to crops.
A knowledge of insect pests would be necessary in order to breed
insect resistant varieties, and to protect susceptible breeding materials
from pest damage.
Bacteriology. Legumes have root nodules containing Rhizobium,
which fix atmospheric nitrogen. The efficiency of this system depends
upon both the host and Rhizobium genotypes. Therefore, in legume
improvement a knowledge of Rhizobium would be helpful. This
aspect of legumes is receiving a great deal of attention these days.
Introduction to Plans Breeding
5
Plant Biochemistry. Several types of quality tests are required 3
to determine the quality characteristics of a crop variety. These \
tests often involve chemical analyses, e.g. f protein content, amino 1
acid content etc. A knowledge of biochemistry would be helpful in
conducting these tests, and also in developing selection techniques
for such characters. 54
Statistics. For a precise comparison of performances . of various
varieties, a sound knowledge of statistical methods is a must. The
breeder must be well- versed in field plot technique, experimental
designs and relevant statistical analyses and tests. The understanding
of quantitative inheritance is also based on statistical principles.
In addition to the above, the breeder must be aware of the
present market demands, needs of the farmer and the problems of
crop production in the concerned area. He should also be able to
see into the future to be able to meet the challenges that the farmers
may face years later. The breeder has to plan several years ahead
because it takes at least 12-15 years to develop and release, a new
variety.
It is difficult for one person to be specialist in all the above
areas. As a result, modern plant breeding is becoming more and more
of a team effort. Specialists in genetics, pathology, entomology and
agronomy cooperate with the plant breeder in crop improvement.
The Indian Council of Agricultural Research (ICAR) has
formulated its All India Coordinated Crop Improvement Projects
on these lines. -
OBJECTIVES OF FLAW BREEDING’
Plant breeding aims to improve the characteristics of plants
so that they become more desirable agronomically and economically .
The specific objectives would vary greatly depending on the crop
under consideration. Some of the main objectives of plant breeding
may be summarised as follows.
Higher Yields. Most of the breeding programmes aim at higher
crop yields. This is achieved by developing more efficient genotypes,
£*&*» hybrid varieties of maize (Z. mays), sorghum (S. bicolor ), bajra
(P> americanum ), etc.
Improved Quality. The quality of plant .produce determines its
•suitability for various uses. Therefore, quality is an important
aspect for plant breeders. The quality characters vary from one crop
to another, eg., grain size, colour, milling and baking quality in
wheat (Triticum mestimm ) ; cooking quality-in mtfOrym 'sative) ;
malting in barley (ffordeum vulgare) ; size, colour md flavour- of
6
Plant Breeding : Principles and Methods
fruits ; keeping quality of vegetables ;■ protein content in cereals
' and . legumes ; lysine content in cereals ; methionine and tryptophan
contents in pulses etc.
Disease and Insect Resistance. Resistant varieties offer the cheapest
and the most convenient method of disease and insect -control;
In some cases, they offer the .only feasible means of control, e g.,
ruses in wheat (T. aestivum ). Resistant varieties not only increase
production but also stabilise it.
Change in Maturity Duration. It permits new crop rotations and
often extends the crop area. Development of wheat varieties suitable
for late planting has permitted rice-wheat rotation. Thus breeding
for early maturing crop varieties, or varieties suitable for different
dates of planting may be an important' objective.
Agronomic Characteristics. Modification of agronomic cha-
racteristics, such as plant height, tillering, branching, erect or trailing
habit etc., is often desirable. For example, dwarfness in cereals Is
generally associated with lodging resistance and fertilizer responsive-
ness,
, Photpinsensitivity. Development of photoinsensitive and tempera-
ture insensitive wheat and photoinsensitive rice (O. sativa ) varieties
has permitted their cultivation in new areas.^ Rice is now cultivated
in Punjab, while wheat is a major rafct crop in West Bengal.
Synchronous Maturity. It is highly desirable in crops like rnung
(Vigna radiata), where several pickings are necessary.
Nonshatteriag Characteristics. It would be of great value in a crop
like mting.
Determinate Growth. Development of varieties with determinate
growth is desirable in crops like mung, pigeon pea (Cajanus cajan) s
cotton ( Gossypium sp .), etc.
Dormancy. In some crops, seeds germinate even before harvesting
if. there are rains at the time of maturity, e.g u mung, barley
(if. vulgare), etc. A period of dormancy In such cases would check
the loss due to germination. In some other cases, however, it may
be desirable to remove dormancy.
Varieties- for New Seasons. Traditionally maize is a kharif crop.
But scientists are now able to grow maize (Z.mays) as. rabi and
zaid crops. Similarly, mung is grown as a summer crop in addition,
to the main kharif crop.
Moisture Stress and Salt Tolerance. Development of varieties
for rainfed areas and for saline soils would be helpful in increasing
crop production in India. The major proportion (Ca. 70%) of the
cropped area in the country is rainfed. The estimates of the salf-
affected (saline) soils in the country vary from 7 to 20 million
hectares, of which about 2.8 million hectares are alkaline soils.
'Most of these areas are spread in the” states- of Utter Pradesh*
Haryana and Punjab. '
Introduction to Plant Breeding
Elimination of Toxic Substances. Some crops have toxic subs-
tances which must be eliminated to make them safe for consump-
tion. For example, khesari {Lathyrus salivas) seeds have a neuro-
toxin, p-N-oxafy famine alanine (BOAA) that causes paralysis.
Similarly, Brassica oil has erucic acid which is harmful to human
health. Removal of such toxic substances would increase the nutri-
tional value of these crops.
Winter Hardiness would be desirable in certain situations.
DECCA;
Fig. 1.2. Cobs of Improved, high yielding maize hybrids,
(Courtesy ICAR). , : -'S
ACTIVITIES IN PLANT BREEDING
The desired changes in the 'genotypes of crop species mi the
consequent benefits to the farmers are brought about by a -series df
interrelated’’ and largely interdependent activities. These activities'
are : creation of variation, selection, evaluation, multinlieation
8 Plant Breeding : Principles and Methods
distribution (Fig, 1.3). Creation of variation is the prerequisite for
HYBRIDIZATION
1. INTER VARIETAL
2. DISTANT
DOMESTICATION M
MUT
•CREATION
germplasm
COLLECTION
POLYPLOIDY
L AUTO*
2. - ALLO*
VARIATION
INTRODUCTION
■f SOMOCLONAL
| VARIATION
NATURALLY
EXISTING
VARIABILITY
CREATION
SELECTION
EVALUATION
MULTIPLICATION
• DISTRIBUTION
Fig. 1.3,. ' Activi.fes. in plant breeding,
'my plant breeding programme ; there can be no improvement in the
absence of variation. Domestication, germplasm collection, introduc-
tion, hybridization (both intervarietal and distant), mutation, poly-
ploidy and soraoclonal variation are the chief means for generating
variation. Selection attempts to isolate the most desirable genotype
from the mixture of numerous genotypes in a population. Selection
has to be based on phenotype of the plants. Generally, there is highly
variable, correspondence between the phenotypes and the genotypes
of plants. The various selection schemes attempt to overcome this
introduction to Plant -Breeding
9
difficulty and are designed for specific situations, e.g., for self md
cross-pollinated species etc. Evaluation concerns with the comparison
of performance^ newly evolved genotypes (strains/selections) with
that of the existing varieties. These comparisons are made for two
or more years, preferably at several locations, to obtain an estimate
of stability in the performance of new strains. The all India Co-
ordinated Crop Improvement Projects of ICAR perform this func-
tion in an excellent manner. If a newly developed strain is superior
to the existing varieties, it is released as a sew variety for general
cultivation on a commercial scale. The benefits from new variety
can be obtained only when the new variety is multiplied and distri- .
buted to the fanners. These two functions are performed by various
seed agencies in the country.
. for an efficient crop improvement programme, the above
activities have to be properly coordinated and efficiently geared to
maximise the outputs from the; programme. A deficiency or slackness
In any one of these steps will definitely reduce the efficiency of the
programme. This will lead to a wastage of valuable resources,, which
no. nation, particularly a developing nation, can afford. Therefore, a
regular and critical review of the progress and the problems has to'
be 'made, the deficiencies in the various activities have to be identic
ied and appropriate remedial measures have to be taken to ensure
continued progress in breeding programmes.
SOME IMPORTANT ACHIEVEMENTS
The present-day crop plants are very different from me wild
species, mostly weeds, from which they originated. This change has
been brought about by man through plant breeding. The extent of
this change is often so great that it is difficult to realise that the
cultivated plants did come from the weedy wild species. This can
be appreciated by comparing the cobs of a local variety of maize
(Z. mays) with those of hGanga- 10T, .an improved maize variety
{Figs. 1,2, 1,4). Clearly, the present-day crop plants were virtually
reconstructed by man from weedy wild plants.
. , One of the most important developments of modern 'agriculture
has been the production of semidwarf cereal varieties, particularly of
wheat and rice. The semidwarf wheat {T. aeslivum) varieties were
developed by. N.E. Borlaug and his associates at CIMMYT (Inter*
national Centre for Wheat and Maize Improvement), Mexico. They
used a Japanese variety Norm 10 as the source of dwarfing genes. In
1963, ICAR introduced several dwarf selections from CIMMYT.
K.alyan Sona and Soaahka were selected from these materials. For
more than one decade, these varieties were the most popular wheat
■ varieties-' in .India. A .great, majority of the wheat varieties: now ■ ■
grown in the country are semidwarf. ' These semidwarf wheat varie-
ties are lodging resistant, fertilizer responsive and high., yielding*
They are; 1 generally, resistant ■ to rusts and other major diseases' of .
10
Plant Breeding ^ : Principles and Methods
wheat dm to incorporation of resistaoce genes m their 'genotype.
This has greatly increased and stabilised wheat production m the
country. These varieties are photoinsensitive and many of them are
suitable for late planting. This has enabled" cultivation of wheat in
nontraditional areas like West Bengal
Fig. I A. Cobs of a local open-pollinated variety of maize from A.P.
, (Courtesy, ICAR)
Similarly, the development of semidwarf rice (O. saliva) varie-
ties has revolutionised rice cultivation. These varieties were derived
from Dee-geo- woo-gen, a dwarf, early-matu ring variety of japonka
rice from Taiwan. Taichung NatiyO (T?fl\ developed in Taiwan,
and !R/8,_ developed ajtlKRI ’(International Rice Research Institute),
PBllIppmes^were introduced in India in 1966/ They were extensively
grown for few years, but were later replaced by superior semidwfurf
varieties developedJrLJndia, e.g., Jaya, Katna etc. The semidwarf
rice varieties are lodging resistant, fertilizer responsive, high yielding
and photoinsensitive. Photoinsensitivity has allowed rice cultivation
in nontraditional areas like Punjab. Even in traditional areas, dee-
wheat rotation has become possible only due to these varieties.
Another noteworthy achievement is noblisation of sugarcane.
The Indian canes were of Saccharum barberi origin and were largely
grown in North India. They were hardy, but poor in yield and
introduction to Plant Breeding
sugar content. The tropical . noble canes of Seeker um officinarum
ongm had thicker stem and higher sugar content. But they perfor- .
med badly m North India primarily due to low winter temperatures
in this region. C. A. Barber, T.S. Venkatraman and otheis at the
Sugarcane Breeding Institute, Coimbatore, transferred the thicker
stem, higher sugar content and other desirable characters from the
- noble canes to the Indian canes. This is commonly referred to as-
no blisation of Indian canes . They also crossed Sac char urn spent aneum
(vern. kans), a wild species, to transfer disease resistance and other
desirable characteristics to cultivated varieties. Several high yielding
varieties with high sugar content and well adapted to local climate
have resulted from this breeding programme. At present, sugarcane
breeding all over the world is based on the rsoblisation technique.
The development of hybrid varieties of maize (Z. mays), jowar
bicolor) and bajra (P. americanutn) deserves a special mention. A
programme to develophybrid maize began in India about two decades
ago in collaboration with Rockefeller and Ford Foundations. Seve-
ral hybrid varieties have been released since then. The Ganga series
of hybrids, e.g., Ganga Safed 2, and Deccan, are a few examples.
Similarly, several hybrids have been released in jowar, e.g CSH L
CSH 2, CSH 3, CSH 4, CSH 5, etc., and in bajra, e g., FEB 10,
PHB 14, BJ 104 and BK 560. For certain reasons, hybrid maize
varieties coaid not become popular with the farmers. But in some
states like Karnataka, hybrid varieties occupy large areas. More-
recently, composite varieties are being developed to overcome the
difficulties encountered with the hybrid varieties. For example,
Manjari, Vikram, Sona, Vijay and Kisan are some of the notable
maize composite varieties. Some recently released composites are :
LG 1 , NLD, Renuka, Kanchan, and Diara 3. The composite
varieties often yield as much as the hybrid varieties and do not have
the drawbacks of the latter. More • notably, the farmers need not
replace the seed every year in the case of composite varieties.
India has achieved the distinction of commercially exploiting
heterosis in cotton. The first hybrid variety of cotton was H 4 (a
hybrid from two G. hirsutum strains) ; it was developed by the
Gujarat Agriculture University (Surat Station) and released for
commercial cultivation in 1970. Since then, several other hybrid
varieties, e.g., Jk Hy 1, Godavary, Sugema, H 6 and AHH 468 (all
within G. hirsutum), Varalaxmi, CBS 1 56, Savitri, Jayalaxmi and
H2HC (all G. hirsutum X G. barbadense), have been released for culti-
vation. The hybrid varieties are high yielding, and have high
ginning outturn and good fiber quality. They are becoming
increasingly popular with the farmers ; according to an estimate,
they occupy about 70% of the total area under irrigated cotton (this
came to about 1.5 million hectares in 1 985-86). It is noteworthy
that the farmers are more than willing to pay the high cost of the
hybrid seed (around Rs 50.00 per kg), which is produced by hand
emasculation and hand pollination. Efforts are being made to utilize
male sterility for hybrid seed production ; this is expected to reduce
the seed cost.
p plant Breeding : Principles and Methods
T . . nn » v a [ e w examples of the achievements ot plant
breeding
ting local varieties , olten tne J Jems a ^ ^ the cost
(.2) due to resistance to diseases the imp - eIds In addition,
of plant protection ; they j^ d !S££ (consider the points listed
(3) they offer numerous other advantages tconsiuei v of
India Coordinated Crop Improvement Irojects for most
important crops ( Chapter 28, Tab.e z8J).
UNDESIRABLE CONSEQUENCES
In ? eneral improvement in the yield of a crop species is accom-
panied with a reduction in variability among the cuUivated vjnu«*
of that soecies. This reduction is produced by two potent taao-s •
first reoiarement of heterogeneous local varieties by *ew domtnan ,
v-i-ieties as Dare fits m breeding programmes. Ihc aop rovca
varieties are cLmo P nly purelines (self-pollinated species) or hybrids
(cross-pollinated species), which are much more homogeneous than
the unimproved local and open-pollinated varieties. A few improved
varieties become predominant and rapidly replace the loca,. varieties
leading to a rapid depletion of genetic variability i&nette eroswnl
This, in turn, limits the prospects of further improvement s ^
species since variability is the prerequisite for arwmodmcatioa m
the characteristics of a crop species. Germplasm collections aim at
minimising the detrimental effects of genetic erosion by collecting
and preserving the variability in crops and their related species.
Many of the improved varieties have one or more paints
(immediate or somewhat removed in the ancestry) in common with
each other. For example, many semidwarf varieties of rioe -have
IR 8 or TN 1 as one of their parents.; IR 8 and IN 1 are related to
each other by the common parent Dee-geo-woo-gen, the source of
their semid waring gene. Similarly, almost all the semMwarf wheat
varieties have Rht i* Rht 2 or both these genes for reduced height ;
these genes have been derived from a single wheat variety ..Norm 10.
Thus the improved varieties of a crop species are becoming increa-
singly similar to each other due to the commonness of one or more
parents in their ancestry. Tnis has led to. the narrowing down of the
genetic base of these varieties ; genetic base refers to the genetic
variability among the varieties of a crop species. The narrow genetic
base has created genetic vulnerability , which refers to*siisceptibiiity
of most of the . cultivated varieties of a crop species to a disease,
insect .pest or some other -stress due to the s*»Haiiity4n timigepe-
types. An example of genetic vulnerability «« the OB®k in
Introduction to. Plant Breeding
epidemic proportions^? Helmintkosporium (now, Cochliobolm ) Ifeaf
blight of maize in the 1970s in U.S.A. This occurred due to the
4 extreme susceptibility of most of the commercial hybrids to the leaf
f blight ; this susceptibility was produced by the Texas male sterilitv
cytoplasm present in these hybrids. A similar case in India is the
susceptibility of early bajra hybrids to downy mildew and ergot ;
again this susceptibility was contributed by the male sterile parent of
these hybrids, Tift 23 A or its dwarf derivative, Tift 23D a A. But in
this case, the susceptibility was due to some nuclear genes aEd not
due to the male sterile cytoplasm. Genetic vulnerability can be
avoided by using diverse and unrelated parents in breeding program-
mes, and by using unrelated sources of male sterility, semidwarfaess
etc. Breeders are becoming increasingly aware of this problem and
, they are making conscious efforts to broaden the genetic base of the
t £- cultivated varieties.,
Another problem has been generated by the lopsided empha-
sis oa breeding for resistance to the major diseases and insect pests
only.' This has often resulted in an increased susceptibility to thus
far minor diseases. As a result, these diseases have gained in impor-
tance and, in some cases, produced severe epidemics. A case in point
is the epidemic caused by Botrytis cinerea (grey mold) in chickpea
during 1980-83 and 1981-82 crop seasons in Punjab, Haryana and
parts of U.P. and Biban Another example is the severe infection by
Karnal bunt (TiUetia sp.) oa some wheat varieties, e.g., WL 711.
This problem is difficult to overcome since resource constraints may
never permit a breeder to screen his material against al! the diseases
and pests of a crop species. We may probably have to, so to say,
Y learn to live with this problem, and tackle it as and when it arises. ‘
"it Sooner or later, the variability for yield in a crop species is
I exhausted and no further yield increases are obtained through breed-
} ing, i.e., the yield reaches a plateau. Such plateaus were evident in
wheat and rice yields before the exploitation of semidwarfing genes,
and are being, experienced once again. In such cases, new variability
has to be introduced in the breeding programmes in order to break:
the plateau. There is an increasing evidence that wild relatives of
cultivated species may provide ‘yield genes’ for breaking the yield
plateaus. In some cases, a novel breeding approach may be useful in.
raising crop yields beyond the plateau, e.g., hybrid rice.
>
FUTURE PROSPECTS
The past achievements of plant breeding fully illustrate its
future possibilities. The improvements made in the crop plants so far
represent only a small portion of the possible improvements. There is
considerable scope for further modifying the present-day crop species.
It is believed that the genetic makeup of the plants may be modified
to a much greater extent than we normally appreciate. Further,
breeding of several crop plants, like pulses and oilseeds, has not been
so intensive as that of wheat and rice. Much improvement in yields
and other characteristics can be made in these crops. Plant breeding,
ill
14
Plant Breeding : Principles and Methods
Ifl
-
together with improved crop management practices* is the only
answer to the ever increasing demand for foodgrains.,
SUMMARY
Plant breeding is a silence based on principles of genetics and cytogene-
tics. It aims at improving the genetic makeup of- the crop plants. Improved
varieties are developed through plant breeding. Its objectives are to improve
yield, quality# disease resistance, drought and frost tolerance aod other charac-
■tensile^ of the crops. Plant breeding has been crucial in increasing agricultural
production. Some ^ well known achievements are development of ' semidwarf
wheat and rice varieties, noblisation of Indian canes, and production of hybrid
and composite varieties of maize, jowar and bajra . Plant breeders should be
able to make similar contributions in the future as well.
QUESTIONS
. L Briefly discuss the various factors responsible for low agricultural pro-
ductivity in India. Explain the contributions of plant breeding in Improving
the agricultural productivity.
■2. List the contributions of the following scientists : (II Thomas Fairchild,
'»> Joseph Koekeuter, (iii) Andrew Knight, (iv) l& Goatem and Shireff,
Cv) Vilmofin Ivi) Niisson-Ehle, (vii) iohannsen, (viii) Q.L Mendel,
and (ix) G.H. Shull
3» Describe the various objectives of plant breeding with the help of suitable
examples.
4. Describe its brief how plant breeding has helped in improving the pro-
f “ foito * iQg cr °Pf « * (0 Wheat, (ii) Rice, (iii) Maize,
ffcrough pJwt bfwdioS' SCUSS £h * f “ tUre prospects of improviD 8 «op plant*
5. Briefly describe the history of plant breeding. Discos* the nature of plant
a ® d briefly outline the disciplines with which a plant breeder
ought to be familiar for effective crop improvement.
Suggested Further Reading
AlL New York' m °' Pr5BCip!es of Piant Breeding. John Wiley and Sons., Inc.,
M-T.a. r Early history of plant breeding in India. Indian J. Genet.
Pal, B.P. 1958. Progress of Plant Breeding in India. Indian J. Genet. 18 : 1-7
Poehlman J.M. and Borthakur, D.N. 1969. Breeding Asian Field Crops
New DelhT. Re,erence fo Crops of India - Oxford and IBH Publishing Co.!
Simmonds, N.W. 1973. Plant Breeding. Phil. Trans. Royal Soc. Lond.B, 267 :
S,MM New’Yor V k: ^ Pfiacip !es ^ of. Crop Improvement. Longman. London and
CHAPTER 2
Domestication, Plant Introduction
and Acclimatisation
DOMESTICATION
Domestication is the process of bringing wild species under
human management . The present-day cultivated plants have been
derived from wild weedy species. The first step io the development
of cultivated plants was their domestication. Most of the crops
were domesticated by the prehistoric man. Knowingly or un-
knowingly he must have selected for the characteristics that made
the plants more suited to his needs. Under domestication, the crop
.species have changed considerably as compared to the wild species
from which they originated. The change is often so great that they
are .classified as distinct species. Asa result, in many, cases, the
parental , wild species of the cultivated plants are not definitely
known. ' This great difference between cultivated plants and their
wild relatives was brought about by selection by man as well as
nature. The domesticated species were selected for characteristics
entirely different from those for which the wild species were selected
in nature. Therefore,' the two groups of plants developed in two
different, often opposite, directions.
Domestication of wild- species is still being done and is likely
to continue for . a. long time in the future^ This is because the human
needs are likely to change with time. Consequently, the wild species
of little importance today may assume great significance tomorrow.
This is particularly true for microorganisms producing antibio-
tics, involved in nitrogen fixation, and producing other compounds
of industrial or medical interest ; forest trees producing timber and
other commercial products ; medicinal plants ; and plants fulfilling
specific needs. A notable case of recent dometication vis that of
several members of Euphorbiaceae producing latex. The latex of
these plants may be commercially used for extraction ,of petroleum
products, including petrol and diesel. A large scale cultivation of
these plants is being done in U.S.A. and Japan. The Department
of Science and Technology, Government of India has initiated a
16
Plant Breeding : Principles and Methods
project for cultivation of jojoba (Simmondsia sp.) in arid zones of
Rajasthan, Gujarat, Maharashtra and Uttar Pradesh* Seeds of jojoba
contain oil which is comparable to sperm whale oil and is highly
suitable as an industrial lubricant. The plants producing latex are
gopher plant (Hevea sp.), milkweed (Euphorbia latkyrus) etc. They
are hardy desert plants and their latex compares favourably with
petroleum crude, and is being used for extraction of petroleum
products. As a result, fields of these plants are often, called living
oil fields.
Natural and Artificial Selection- under Demestleation
The precis® sequence of events in evolution of' crop plants-
under domestication is not known. But it may be generalised that
in the early stages a considerable •variability existed in tbs domesti-
cated species., New variability arose from hybridization followed
by recombination, and from mutation. The extent and manner of
selection exerted by man on the domesticated species is not known.
Selection may be described as a phenomenon of some -genotypes from
a population leaving behind more progeny than others. Tbs. genotypes
that produce more progeny are selected for and the others are selected
against. la aaturej there "is a continuous selection by natural forces,
s,g., temperature, soil, weather, pests, diseases, etc. As a result, the
genotypes more suited to a given environment have behind more
progeny than the less adapted ones. This process is known as natural
selection. Thus natural selection is one of degree and it rarely stops
a genotype from producing some progeny. The selection by man
(artificial selection), on the other hand, often permits only the select-
ed plants to reproduce ; the progeny from the remaining plants are
generally discarded. Thus natural selection retains considerable
variability in the _ species, while artificial selection progressively
reduces this variability.
There is no doubt, however, that man exerted considerable
■ selection on domesticated plant species. It may be expected that he
would have selected those plant types that best suited his needs. It
is likely that he selected for larger fruit and grain sizes. It may he
expected that his methods of selection were primitive, in that they
were not based on scientific principles. His sole criterion of selec-
tion would have been the phenotype of plants. But there could
be no doubt that this, combined with the natural selection under
domestication, was highly effective. Artificial and natural selections
have ied to several distinct changes in the characteristics of domesti-
cated species.
Changes in Plant Species under Domestication
Almost all the characteristics of plant species have been
affected under domestication. The characters that show more
distinct changes are those that have been objects of selection and
are still plant breeding objectives in many cultivated species. Some
of the important changes that have occurred under domestication
are briefly listed below.
Domestication Plant Introduction and Acclimatisation
1. Elimination of or reduction in shattering of pods, spikes, etc.
This change has taken place in most of the cultivated species.
% Elimination of dormancy has taken place in several crop species.
Lack of dormancy has become a problem in crops like' barely
(II vulgare), wheat, (T. aestivum ), mung (Vigna radiata ) etc.
3. Decrease in toxins or other undesirable substances. The bitter
’principle of cucurbitaceous plants is an example.
4. Plant type has .been extensively modified. The cultivated plants
• show altered tillering, branching, leaf characters, etc.
5. In several crop 'Species, there has been a decrease in plant height,
e.g>, cereals, millets. This Is often associated with a change from
' indeterminate to determinate habit.
6. In some species, on the other hand, there has been an increase in
plant height under domestication, e.g., jute (Corchorus sp.),
sugarcane (SV officinamm ), forage grasses etc.
7. Life cycle has: become shorter in case of some plant species. This
is particularly so in case of crops like cotton (Gbssypium sp.),
arhar (Cajimus cajan ), etc.
8. Most of the crop plants show an increase in size of grains or
fruits. . ■ v ;;
9. Increase in the economic yields is the most noticeable as well as
desirable change under domestication. This is self-evident in
every crop species. .V:L7>;
10. In many crop species, asexual reproduction has been promoted
under domestication, e g., sugarcane, potato (Solanum tubero-
sum), sweet potato (Ipomoea batatas), etc.
IL There has been a preference for polyploidy under domesti-
cation. Many of the domesticated plant species are polyploids,
e.g., potato, wheat, sweet potato, tobacco Nicotian a sp., etc.,
while diploid plants are present in nature. Thus domestication
seems to have favoured polyploidy in these crop species.
12. In many species, there has been a shift in the sex form of the
species. In many dioecious fruit trees, bisexual forms have
developed under domestication. Self- incompatibility has also
been eliminated In many crop species.
13. Variability within a variety has drastically decreased under
domestication.' The extreme case is that of pureline varieties
which are completely homogeneous genotypically.
PATTERNS OF EVOLUTION IN CROP PLANTS
It is apparent that selection by 'nature and man has been
responsible for evolution of crop plants. Wnwever, selection is
18
Plant Breeding : Principles and Methods
effective in altering a species only when variability exists in the
populations of that species. There are three major ways in which
variability has arisen in the crop species. The patterns of evolution
of various crops may be broadly classified according to the mode of
origin of variation crucial for evolution of that species.
Mendelfan Variation. Many crops have evolved through variation
generated by gene mutations, and by hybridization between
different genotypes within the same species followed by recombina-
tion. Ultimately, all the variability in any species originates from
gene mutations. Most of the gene mutations are harmful and are
eventually eliminated. But some mutations are beneficial and are
retained in the population. The mutations may be grouped into
two categories ; macromutation and micromutation. A macromutation
produces a large and distinct morphological effect , and often affects
several characters of the plant . A single macromutation has led to
the differentiation of modern maize (Z. mays) plant from the grassy
pod corn. This mutation has affected the position of male and female
inflorescences, the habit of the plant and several other characters.
Similarly, cabbage ( Brassica oleracea ), cauliflower (B. oleracea)*
broccoli (B. oleracea), and Brussel’s sprouts {B. oleracea) have
originated from a common wild species and they differ from each
other with respect to a few major genes.
The greater part of variation, however, has resulted from
mutations with small and less drastic effects , Le., micromutations . .
Since micro mutations have only small effects, they tend to be accu-
mulated in a population. Natural selection would accumulate and
select for more favourable gene combinations. Man would have
selected from the populations desirable plant types leading to
differentiation ©f the domesticated species from the wild ones.
Several important crops have evolved through Menddian variation,
e.g., barely, rice beans ( Phaseolus sp.), peas {Pisum sativum ),
tomatoes (Lycopersicon esculent um), linseed {lAnum usitatissinmm\
jowar, bajra and many other crops.
Interspecific Hybridization, interspecific hybridization refers to
crossing of two different species of plants . The resulting Fa is generally
more vigorous than the parents But segregation in and later
generations produces a vast range of genotypes. This is because the
parental species are likely to differ from each other for a large number
of genes. Most of the recombinants in the segregating generations are
likely to be weak and undesirable. Often interspecific hybrids are
highly sterile and do mt set seeds. There is little evidence to suggest
that interspecific hybridization contributed to any g^eat extent in
evolution of crop species.
But in some cases, the interspecific hybrids may have repeatedly
backfcrossed to one of the parental species. Thus most of the geno-
type of that parental species would be recovered alongwith few or
several genes from the other parental species. This process is known
as introgressive hybridization and leads to the transfer of some genes
Domestication , Plant Introduction and Acclimatisation
from one species into another. The modern maize (Z. mays) is
postulated to have developed through introgressive hybridization
between the primitive maize and a wild grass, Tripsacum. It is
supposed that some genes from Tripsacum were transferred to the
primitive maize which resulted in the modern maize.
Interspecific hybrization has led to the development of several
strawberry varieties. The F 1 from a cross between two species of
strawberries, Fragaria virginiana and Fragaria chiloensis , was back-
crossed to the two parent species to produce many varieties of
commercial value. In certain fruit trees, such as pears, plums,
cherries and grapes, and ornamentals, e.g. f irises (Iris sp.), roses
(Rosa sp.), lilies (Lillium sp.) etc., vegetative propagation is
commonly used. In such species, many varieties are interspecific
hybrids (F 3 ).
Polyploidy. Generally, autopolyploidy leads to increased vigour,
larger flowers and fruits etc. over the diploid forms. Many varieties
of ornamental plants are autopolyploids. The commercial banana
(Musa paradisiaca) is an autotriploid (3*) ; it has larger and seedless
fruits in comparison to the diploid banana. Triploid varieties are
known in apples (Pyrus malus ), watermelons (Citrullus vulgaris ),
sugar beets (Beta vulgaris) and some other crops. The commonly
grown potato (Solanum tuberosum) may be regarded as an autotetra-
ploid, although interspecific hybridization may also be involved.
S. tuberosum has 3x, 2x and 4x types. Some of the 2x progeny
obtained from the 4x potato are fully fertile and as vigorous as the
4x types, indicating that it is largely an autotetraploid. Other
autopolyploid crop species are sweet potato (6x), oat (A vena abyssi-
nica , 4x) and alfalfa (Medicago sativa , 4x). Thus autopolyploidy has
played a limited role in crop evolution.
Allopolyploidy, in contrast, has been considerably more
important in crop evolution. Allopolyploidy results from chromo-
some doubling of interspecific F 3 hybrids. About 50 per cent of the
crop plants are allopolyploids. Some of the important allopolyploid
crop plants are wheat, tobacco, cotton, sugarcane, oats (Avena sp.),
tai ( Brassica juncea ), rapeseed (Brassica napus) etc. Origins of wheat,'
tobacco, cotton, and oats have been extensively investigated.
Common bread wheat (Triticum aestivum) is an allohexaploid, while
cotton (Gossypium hirsutum and G. barbadense) and tobacco
(Nicotiana tabacum and N. rustica) are allotetraploids. There is
evidence to suggest that N. tabacum originated from chromosome
doubling of the FY hybrid from N. syhestrisxN . tomentosa , Triticale
is a mm made allopolyploid developed by chromosome doubling of
the Fi between rye (Secale 'cere ale) and tetrapioid or hexaploid
wheats. Triticale has shown much promise particularly in areas of
moisture or temperature stress.
CENTRES OF ORIGIN
There 1$ considerable evidence that the cultivated plants were
not distributed uniformly throughout the world. Even today.
20
Plant Breeding : Principles and Methods
certain areas show far greater diversity than others in the forms of
certain cultivated crops and their wild relatives. NX Vavilov .
proposed that crop plants evolved from wild species in the areas *
showing great diversity and termed them as primary centres of origin.
Later, crops moved to other areas primarily due to the activities of
man. These areas generally lack the richness in variation found in the
primary centres of origin. But in some areas , certain crop species
show considerable diversity of forms although they did not originate
there . Such areas are known as secondary centres of origin of these
species .
The concept of centres of origin was given by Vavilov based
on his studies of a vast collection of plants at the Institute of Plant
Industry, Leningrad. He was director of this institute from 1916 till
1936. He also postulated the Law of Homologous Series in Varia-
tion. This law states that characters found in one species also occur in f
other related species. Thus diploid (2x), tetraploid (4x) and hexa-
ploid (6x) wheats show a series of identical contrasting characters.
Similarly, genus Secale duplicates the variation found in Triticum.
Thus a character absent in a species, bat found in a related species,
is likely to be found in the collections of that species from the centre
of its origin.
Eight main centres of origin are recognised as proposed by
Vavilov : China, Hindustan, Central Asia, Asia Minor, Mediter-
ranean, Abyssinia, Central and South America.
The China Centre of Origin,, This centre consists of the mountaine-
ous regions of central and western China and the neighbouring low-
lands. It is the largest and the oldest independent centre of origin. %
The crops that originated in this area { primary centre of origin) *
are : soybeans ( Glycine max), radish ( Raphanus sativus \ Colocasia
antiquorum (bunda), Panicum miliaceum (proso millet), and some
other, species of millets, buck wheat ( Fagopyrum escluentum), Papaver
somniferum (opium poppy), several species of Brassica and Allium ,
Solanum melongena (brinjal), some species of Cucumis and Cucurhita ,
pears (Pyrus communis)* peaches {Prunus persica ), apricots {Prunus.
armeniaca), plums ( Prunus divaricata ), orange ( Citrus nobilis),
Chinese tea ( Camellia sinensis ) and naked oats (Aveda nuda).
In addition, it is secondary centre of origin for several crop
plants, e.g. 9 Zea mays (maize), Phaseolus vulgaris (rajma), cowpea
(Vigna anguiculaia ), turnip ( Brassica rapa) and sesame or til
(Sesamum indicum). .
The Hindustan Centre of Origin. This centre includes Burma, Assam, % %-
Malaya Archipelago,. Java, Borneo, Sumatra and Philippines, but
excludes North-west India, Punjab and North-western Frontier
Provinces. It is the primary centre of. origin of rice (Oryza saliva ),
arhar (Cajanus cajan ), gram or chickpea ( Cicer arietinum ), cowpea
(V. anguiculaia ), mung (V. radiata, ), brinjal ( S . melcigena ), Cucumis
sativus (cucumber), Lactuca indica (Indian lettuce), certain species
■
92 plant Breeding : Principles and Methods
or
mwjfr-a), banana (Masa sapientum) and turmeric (.Cwrca/n
domestica). _
The Central Asi* Centre of Origin. It includes North-west India
to :,k Th? North-west Frontier Provinces and Kashmir), all ot
Austin the Sovlt ^ Republics of Tadjikistan and Uzbekistan
anH Tian Shan It is also known as the Afghanistan Centre of Origin.
The crops that originated in this centre (primary centre of origin)
Ire • wheat (Triticum aestivum), club wheat (Triticum compactum),
pea («»» sLum), bread bean ( Vida f aba) mmg Oy «
linseed ( Linum usitatissilnum), sesame (S. indicum ), Sdinower
Cerdml tecforte), hemp cotton >
/ n cntivu’f) musk melon (Curcurbita moschata), <*airot (. ua ...us
carota), onion (Allium cepa) garlic
oleracea), pistachio nut (Pistacia vera), & P nc0 ^f2T f/ a Z (Vitis
pear (Pvrus communis), almond (Prunus amygdai f g V
vinifera ) and apple (P. mate). It is secondary centre of origin ot rye
SrAs^Mi^or Centre of Origin. This is also known as the i Near
East or the Persian Centre of Origin. It includes Jj? Tur1™ni-
Minor the whole of Transcaucasia, Iran and Highlands ot 1 unvtnem
stan. The crop species that originated in this region (primary centr
of Origin) include nine species of Triticum, rye (S. cereale) alfaila
(Medicago sativa), Persian clover ( Trifolium resupmatum). carrot
(Daucus carota), cabbage ( Brassica oleracea), oat (Avena sativa),
species of Allium, lettuce (Lactuca sativa), fig (Ficuy awn),
nate ( Punica granatum), apple, several species of \
grape (K. vinifera), almonds (P. amygdalus), chesmuts (CastaneaspA
and P pistachio nut ( P . vera). It is the secondary centre of or f n ,f
rape ( Brassica campestris), black mustard (Brassica nigral , eaf
mustard ( Brassica japonica), turnip (B. rapa) and apricot
(P. armeniaca). ,
The Mediterranean Centre of Origin. Many valuable cereals and
legumes originated in this area. The species that origmated m thi
centre are: durum wheats (Triticum durum), emmer wheats (Triticum,
dicoccum), and other Triticum species, several species of Avena,
barley (Hordeum valgare), lentil (Lens esculenta), several species, of
Lathyrus, pea (P. sativum), broad bean (V. faba), lupins (Lupinus
sp.)/ chickpea (C. arietinum), clovers (Trifolium sp.), vetch (Vma
sativa), several species of Brassica, such as rape and black mustard,
cabbage and turnip, onion (A. cepa), garlic (A. sativum), be
(Beta vulgaris), lettuce (Lactuca sativa), artichoke, asparagus
(Asparagus officinalis), lavender, peppermint (Mentha sp.) and sage.
The Abyssinian Centre of Origin. It includes Ethiopia and hill
country of Eritrea. It is the centre of origin for H. vulgare (barley),
Domestication, Plant Introduction and Acclimatisation 23
Triticum durum , Triticum turgidum, Triticum dicoccum, jowar
(Sorghum btcolor) bajra ( Pennisetum americanum), gram, lentil (L.
esculenta), sem ( Dolichos lablab), pea, khesari ( Lathyrus sativus),
inseed (L. usitatissimum), ^safflower (C. tinctorius), sesame, castor
(Riant, s common,. s), coffee (Coffea arabica), onion and okra (Abelmos-
chos esculentus). It is secondary centre of origin for broad bean
\yicia ' J na),
Central American Centre of Origin. This includes the region of
South Mexico and Central America. It is also referred to as the
Mexican Centre of Origin. The plants that originated here are maize
(Zea vulgaris), lima bean { Phase olus lunatus), melons,
pumpkin (Cucurhita melanosperma ), sweet potato (. ipomoea batata 0,
arrowroot (Canna edulis ), chillies ( Capsicum annuum\ cotton
G hmitmn and G. purpureascens), papaya (Carica papaya ), guava
( Psidium guayava) and avacado (Persea americana).
Tie Souti American Centre of Origin, This centre includes the high
moumainoos regions of Peru, Bolivia, Ecuador, Colombia, parts of
Cltile and Brazil, and whole of Paraguay. The crops that originated
in this centre (primary centre oj origin) are many species of potatoes,
maize, lima bean, peanut (Arachis hypogaea ). pineapple (Ananas
comosa), pumpkin (Cucurhita maxima ), Egyptian cotton (Gossvpium
baroadense), tomatoes, guava, tobacco ( Nicotiana tabacum and other
species} quinine tree (Cinchona calisaya), cassava (Manihot utilissU
may and rubber (Hevea sp.).
. Later, in 1935, Vavilov divided the Hindustan Centre of
Origin into two centres, viz., Indo- Burma and Siam-Malaya — Java
Centres of Origin . The South American Centre was divided into
wree centres, namely, Peru, Chile and Brazil- Paraguay Centres of
Origin, Thus the eight main centres were grouped into 1 1 centres of
A *WP- At the same time he introduced a new centre of origin, the
* Centre of Origin, Two plant species, sunflower ( Helianthus
a'lnuus) and Jerusalem artichoke (Helianthus tuberosus ), oiiginated in
tl \ e U.S.A. centre of origin.
The concept that centres of diveisity represent centres of origin
Isa s been seriously questioned. Plants of a species growing in differ-
ait environments are likely to he different, i.e„ diverse Thus a
species is likely to show a greater variation in a region with varied
at? f 1C ot ^ er ecological conditions. Areas with mountains and
vail*. y$ snow considerable variation in environment Therefore,
pia at species would show a greater variation in such areas. The cent-
res o origin are situated in such mountain-vallev areas. Further the
certreJS oj diversify of many species have shifted with time. This
T CrsIty was brought about by a shift in the area of the
w*tk ^ cul «vation and due to introduction of the species in an area
“ a ^: ater ec °l ° S i ca i diversity. These processes have given rise to
secondary centres of diversity . Consequently, several species have
two or more centres o.f diversity and it is often difficult to determine
whim one of themes tne real centre of origin.
_ i
Plant Breeding : Principles and Methods
Th ... the cen(res 0 f origin may be more appropriately called the
Thus the centr J ° t mav no t be the centres of orgtn
centres of diversity. Thes Q f t ^ e maximum
of Th s serves ls an extremely useful guide
IVer f „ y f eL Sm as to where to search for variation in a given
to plant , e cen t r es of diversity, small areas may
Sp vt?t much greater diversity than the centre as a whole. These
are knol» « 2T™ 1 fSS'
studying the evolution of plants.
PLATS T INTRODUCTION
“/sks rsss sx« su
i» i laryana, of wheat in West
H* of rice iu Punjab. Introduction m«y be classified into
two categories, primary and secondary*
Primtirv Introduction. When the introduced variety is well suited to
52 environment , it is released for commercial cultivation without
nv alteration in the original genotype. This constitutes primary int ™'
diction 7r mary introduction is less common, particularly m countries
hfv u 8 weu“Snised crop improvement programmes Introduction
of semidwarf "wheat (Triticum aestivum) varieties Sonora 64 and
Lerma Roio, and of semidwarf rice (O. sativa) varieties Taichung
S UR 8 and IR 28, IR 36 are some examples of primary
Domestication , Plant Introduction and Acclimatisation , 25
ment of man. Most of these introductions occurred very early in
the history. For example, mung, mustard (Brassica jimcea), pear,
apple and walnut {Juglans regia) were introduced from the Central
Asian Centre of Origin into various parts of India in the dim past
Similary, sesame, jowar, arhar (C. cajan ), Asian cotton (Gossyphm
herbaceum) and finger millet (Eleusine coracana) originated in Africa
and travelled to India in the prehistoric period. It is, therefore,
clear that the plant wealth of various nations is, to a large extent,
the result of plant introductions.
For several centuries A.D., the agencies of plant introduction
were invaders, settlers, traders, travellers, explorers, pilgrims and
naturalists. The plant-introductions were made either knowing!*
or unknowingly. Muslim invaders introduced in India cherries and
grapes from Afghanistan by 1300 A. D. In the 16th century A.'D„
Portuguese introduced maize, groundnut, chillies, potato, sweet
potato, guava, pineapple, papaya, cashewnut { Anacardium occiden-
tals) and tobacco. East India Company brought tea ( Camellia sinen-
sis ), litchi and loquat from China ; cabbage, cauliflower and other
vegetables from the Mediterranean; annatto (Bixa arellana — a source
of edible dye) and mahogany timber tree from the West Indies in the
last quarter of 18th century.
During 19th century, a number of .Botanic Gardens played an
important role in plant introduction. The Calcutta Botanic Gardens
was established in 1781. The Kew Botanic Gardens, England,
arranged introduction of quinine and rubber trees from South America
into India. During and after the last part of 19th century, various
agricultural and horticultural research stations were established in
the country. These stations introduced horticultural and agricultural
plants independent of each other. There was no coordination among
these agencies regarding their introduction activities.
PLANT INTRODUCTION AGENCIES IN INDIA
A centralised plant introduction agency was initiated in 1946 at
the Indian Agricultural Research Institute (IARI), New Delhi. The
agency began as a. plant introduction scheme in the Division of
Botany and was funded by ICAR. In 1956, during the second five
yen r plan, the scheme was expanded as the Plant Introduction and
Exploration Organisation. Subsequently in 1961, it was made, an
independent division in IARI, the Division of Plant Introduction.
The division was reorganised as National Bureau of Plant Genetic
. Resources, (NBPGR) in 1976. However, the nature of activities and
the functions of the bureau have remained the same-; but the scope
and scale of its activities have increased considerably. The bureau is
responsible for' introduce n and maintenance of the germ plasm of
agricultural and. horticultural plants.
; In addition to the National Bureau of Plant Genetic Resources,
.there,, are some other agencies for plant introduction. , Forest ' -
Plant Breeding : Principles and Methods
Research Instil ute, Dehradm, has a plant introduction organisation,
which looks at 'ter introduction, maintenance and testing of germ-
plasm of forest t fees. Thfi Botanical Survey of India was established,
in 1890; if wait responsible for introduction, testing and mainte-
nance of plant materials of ‘botanical and medicinal interest. But at
present, introduction and improvement of medicinal plants is being,
looked after by the NBPGlt. The Central Research Institutes for
various crops, e g' , tea, coffee sugarcane, potato, tobacco, rice e _tc.,
introduce, test and maintain pL ant materials of their interest. But their
activities are coordinated by i he NBPGR, which has the ultimate
responsibility for introduction ac tivities. Plant materials may also be
introduced by indiviiual scientists, universities and other research
organisations. But all the introdut Hons in India must be routed through
the NBPGR, New Delhi.
The National Bureau of Plant Generic Resources. The bureau has its
headquarters at IARI, New Delhi. It bars four substations for
testing of introduced plant materials. These substations represent
the various climatic zones of India. The four substations of NBPGR
are listed below :
j Simla. It is situated in Himachal Pradesh and represents- the
temperate zone ; approximately 2,300 m ab ove sea level.
2. Jodhpur, Rajasthan. It represents the arid zone.
3. Kanya Knmari, Tamil Nadu. It represents tlite tropical zone.
4. Akola, Maharashtra. It represents the mixed’ climatic zone. It
was recently shifted from Amravati.
In addition, a new substation has recently been established at
Shillong for collection of germplasm from North-east India. This
part of the country has a large genetic variability for several crop
species, e.g., rice, citrus, maize etc.
The bureau functions as the central agency for export and
introduction of germplasms of economic importance. The bureau is
assisted in its activities by the various Central Research Institutes of
ICAR. The activities of the bureau are summarised below.
1. It introduces the required germplasm from its counterparts or
other agencies in other countries.
2. It arranges explorations inside and outside the country to collect
valuable germplasm.
3. It is responsible for the inspection and quarantine of all intro-
duced plant materials.
4. Testing, multiplication and maintenance of germplasm obtained
through various sources. This may be done by the bureau itself
at one of its substations or by one of the concerned Central
Institutes of ICAR.
Domestication, Plant Introduction and Acclimatisation 27
5 To supply, on request, germplasm to various scientists or insti-
tutions. The germplasm may be supplied ex-stock or may be
procured from outside in case it is not available in the country.
6 Maintenance of records of plant name, variety name, pro-
pagating material, special characteristics, source, date and
other relevant information about the materials received.
7. To supply germplasm to its counterparts or other agencies in
other countries.
8 To publish its exchange and collection lists Introduction News
Letter containing such lists is be ng published by Food and
Agriculture Organisation (FAO) since 1957 at irregular intervals
NBPGR has also published some lists, and is m the process of
publishing some other catalogues.
9. To set up natural gene sanctuaries of plants where genetic
resources are endangered.
10. Improvement of certain plants like medicinal and aromatic
plants.
PROCEDURE OF PLANT INTRODUCTION
Plant introduction is one Of the oldest, and very effective
methods of plant breeding. The chief function of plant
is to make available variation to be utilized in breeding
programmes. Introduction consists of tbe following steps • procure-
ment quarantine, cataloguing, evaluation, multiplication and
distribution.
Procurement of Germplasm. The scientists in this country submit
their requirements to the NBPGR. If the bureau is unable to meet
the requests from its own stocks or from known sources m the
country, it attempts to obtain thern from its counterparts m other
nurseries Botanic Gardens or from otHer agencies.
Generally, the materials are obtained through correspondence a^s
gifts in exchange of germplasm, in consideration of past gifts from
the bureau or in anticipation of future gifts. Sometimes he
germplasm has to be purchased. bureau whcipates^n Uw
activities of the International Board for Plant Genetic Resources
UBPGR) IBPGR aims at free exchange of plant genetic resources,
E SI. the supply of needed
But sometimes it may be necessary to organise explorations
the required germplasm.
The nlant part to be introduced depends upon the crop
longer viability than other propagules and are packed and trans
2g plant Breeding : Principles and Methods
oorted more easiiv. Other propagules may require special packing
techniques” For example, FAQ packs 4-5 grass tillers ms.de
polythene bags, and the roots of the tillers are surrounded by moist
cotton wool. Techniques are also available for packing other short-
life plant propagules Seeds are generally obtained by surface mad
often ■ packed and sealed in think paper envelopes. Other propagules
and seeds of such species as mango or Cit rus arc shipped by air
mail preferably through commercial airline. Such materials are
personally collated by a representative of the bureau immediately
after it receives information of its arrival from the post office or the
’•airline office
Quarantine. Quarantine means to keep materials in isolation to prevent
spreading of diseases, etc. All the introduced materials are thoroughly
inspected for contamination with weeds, diseases and insect pests.
Materials that are suspected to be contaminated are fumigated
or are given other treatments to get rid of contamination, it
necessary, the materials are grown in isolation for observation ot
diseases, insect pests and weeds. This entire process is known
as quarantine and the rules proscribing them are known as quarantine
rules.
There arc several instances where weeds, diseases and pests
have been introduced alongwith plant materials (see later). This only
emphasizes the necessity for a centralised agency for plant introduc-
tion to ensure effective quarantine of the introduced materials.
All the materials being introduced must be covered by an authentic
phytosanitory certificate from the source country, i.e., they must be
declared free from diseases , weeds and insect pests . . Plant materials
beitnj introduced or exported must conform to certain quarantine
regulations. The quarantine regulations cover plant propagules,
soils, packing materials of plant origin and other suspected objects,
Introductions not conforming to quarantine rules, or suspected to be
contaminated are likely to be destroyed by NBPGR or would be
returned to the source country.
The quarantine control is exercised by the NBPGR at prescrib-
ed ports of entry, e.g., Delhi, Bombay, Calcutta, Madras etc.
The plant materials accompanied by phytosanitory certificates are
thoroughly examined. Materials contaminated by diseases or pests
are: destroyed or returned to the sender. The materials conforming
to the quarantine laws are fumigated against diseases and pests.
Sometimes the introduced materials are grown in isolated nurseries or
green houses for a critical observation on diseases, pests and weeds.
At IARI, units set up in the Divisions of Mycology and Plant
Pathology, and Entomology keep the Bureau’s nurseries under obser-
vation. They assist in evaluation of diseases and pests. This is very
important to prevent entry of diseases and pests and weeds because
they may not only destroy the introduced plants but also the existing
crop species
Domestication, Plant Introduction and Acclimatisation
Restrictions on Export and Introduction of Plant Materials .
Introduction into India. is.. restricted in case ..of. certain plant specif
Qp.tt.oD, berseetn, pnseed and sugarcane seeds .cannot, be - obtained
through. letter post:, Introduction of the following is prohibited :
cocoa and other theobromo species (plant and seed) from Africa,
Ceylon and West Indies ; sugarcane from Australia, Fiji, New
Guinea, Philippines and West Indies ; rubber (plant and seed) from
America and West Indies ; and sunflower from Argentina and Peru.
Entry of unginned cotton and of Mexican jumping beans is strictly
prohibited.
In addition, export of certain plant species is either restricted
or prohibited. Almost all countries restrict exchange of germplasm
of one or the other plant species lndja ijcsttfcfe more,:.:oftfiP,i>rPl>i.
bits, the .exports of propagnles^f tea* jut?, and black peppeyc.
Cataloguing. When an introduction is received, it is given an entry
number, and information regarding the name of the species and
variety, place of origin, adaptation and characteristics are recorded.
The plant materials are classified into three groups.
1. Exotic collections are given the prefix ‘EC’,
2. Indigenous collections are designated as ‘IC’, and
3. Indigenous wild collections are marked as ‘IW\
Evaluation. To assess the potential of new introductions, their
performance is evaluated at different substations of the Bureau. In
case of those crops for which Central Research Institutes are
functioning, e.g., rice, sugarcane, potato, tobacco etc., the
introduced materials are evaluated and maintained by these
institutes. The resistance to diseases and pests is evaluated under
environments favouring heavy attacks by the diseases and pests.
Acclimatisation. Generally, the ■ introduced varieties perform
poorly because they are often not adapted to new environment.
Sometimes, the performance of a variety in the new environment
improves with the number of generations grown there. The process
that leads to the adaptation of a variety to a new environment is
known as acclimatisation. Acclimatisation is brought about by a
faster multiplication of those genotypes (present in the original
population) that are better adapted to the new environment. Thus
acclimatisation is essentially natural selection. Variability must be
present in the original population for acclimatisation to occur. Land
varieties are likely to get acclimatised, while a purline is not likely to
The extent of acclimatisation is determined by (l).the mode of
pollination, (2) the range of genetic variability in the original
population, and (3) the duration of life-cycle of the crop. Cross-
nol’lination leads to far more gene recombinations than seL-polliua-
tion As a result, cross-pollination is much more helpful in accli-
30
Plant- Breeding : Principles and Methods
matisation than self-pollination. As stated earlier,, variability is the
prerequisite for acclimatisation. Therefore, the greater the initial
variability, the more the extent of acclimatisation. Life cycle duration
is important because an annual would produce several generations by
the time a perennial produces oik generation. Each generation
would produce new gene combinations, thereby facilitating acclima-
tisaiion. Mutation plays a role in acclimatisation, but it becomes
important only when the period of acclimatisation is very long.
5 Multiplication ^and Dfetribatioa. Promising introductions or
selections from the introductions may be increased and released as
varieties after the necessary trials. Most of the introductions,
however, are characterised for desirable traits and are maintained
for future use. Such materials are used in crossing programmes and
are readily supplied by the bureau on request.
GERMPLASM COLLECTIONS
The sum total of hereditary material or genes presend in a
species is known as the germplasm of that species. Therefore,
zermolasm collection is collection of a large number of geno types
of a crop species and its wild relatives. Germplasm collection s are
also known as gene banks (or world collections when they are
sufficiently large to include genotypes from all over the world). It
should be kept in mind that regular crossing with the wild relati ves
has played an important role in the evolution of crop plants.
Furthermore, germplasm collections furnish the richest source of
variability. Crop improvement. would ultimately depend upon the
availability of this variability to be utilized in breeding programme s.
With modernization of agriculture, large tracts of land aru
put under pureline varieties of self-pollinated crops and under
m hybrid varieties of cross-pollinated species. This has led to a gradual
| disappearance of local or land varieties (‘desi’ varieties) and open-
" pollinated varieties— both reservoirs of considerable variability.
Cultivation and grazing are gradually destroying many wild species
and their breeding grounds. Wild relatives of crops may be elimi-
nated by introduced species of weedy nature or even by the culti-
vated forms derived from them. The gradual loss of variability in
the cultivated forms and in their wild relatives is referred to as
genetic erosion. This variability arose in nature over a long period of
u'me and, if lost, would not be reproduced in a short period.
Most of the countries are greatly concerned about genetic
erosion. The establisment of IBPGR to coordinate germplasm con-
servation activities throughout the world reflects this concern. Germ-
plasm collections are being made and maintained to conserve as
many genotypes as possible. The germplasm collections contain
land varieties, various wild forms, primitive races, exotic collections
and highly evolved varieties. Some of the important germplasm
collections are listed below.
w
Domestication , Plant Introduction and Acclimatisation 31
L Institute of Plant Industry, Leningrad. It has 1,60,000 entries
of crop plants.
2. Royal Botanic Gardens* Kew, England. It has over 45,000
entries.
3. BeltsviSle, U.S.A., maintains germplasm collections of small
grain crops.
4. World collections of some of the crops are maintained at the
following places.
(i) Sugarcane . Canal Point, Florida, U.S.A. and Sugarcane Breed-
ing Institute, Coimbtore (2,800 entries)
(ii) Groundnut . Bambey, Senegal (Africa)
(iii) Potato . Cambridge, U.K. and Wisconsin, U.S.A.
(iv) Annual New World Cottons . Near Tashkent, U.S.S.R.
(v) Coffee . Ethiopia (Africa)
(vt) Sweet Potatoes . New Zealand.
5. The National Bureau of Plant Genetic Resources, New Delhi,
is maintaining large collections of Sorghum , Pennisetum wheat,
barley, oats, rice, maize and other agricultural and horticultural
crops. For example, groundnut collection is maintained at
Juoagarh, cotton : Nagpur, potato at >Sim!a, tobacco at
Rajahmundhry, tuber crops (other ifhan potato) at Trivandrum
etc.
6. International Rice Research Instfeute (IR3RI), Los Banos,
Philippines, is maintaining 12,000 idee status and varieties.
More than 15,000 entries are maintained at Central Rice
Research Institute, Cuttack.
T. The various International Institutes |Chapter 29) are building
up and maintaining collections ef many species.
Most of the seeds lose viability quickly. Consequently, germ-
plmsm collections have to be grown ?every few years. (1) Growing,
harvesting and storing large coBectious is a costly affair requiring
much time, labour, land and money, (t) There is also risk of errors
in labelling. (3) The genotypic constitution of the entries may also
change, particularly when they are grown in environments consider-
ably different from that to which they are adapted. This is
particularly true in case of cross-pollinated species and for local
varieties of the self-polLnated species. These difficulties may be
considerably reduced by cold storage of seeds. Seeds of most of the
plant species can be stored for 10 years or more at low temperatures
and low humidity. Thus the entries could be grown every 10 years
or .so instead of every one or two years. Cold storage facilities are
being utilized at Fort Collins, U.S.A and at IRRI, Philippines.
NBPGR has developed cold storage facilities for germplasm mainte-
nance ; this is known as National Germplasm Repository . However,
Plant Breeding : Principles and Methods
ot „„ ctnraees of germplasm collections are vulnerable to
low temperature storag ^ no f inal P breakdowns , power .puts, strides
electricity fail (perhaps more importantly) sabotage. In
by electricity stan, an tp p assured continuous power supply
developing f pro blems of long-term storage In addi-
ng by .f^^esS hkely to become targets of attacks during
t,on * Thus i't mav be worthwhile to maintain duplicate collections
r r two separate places, but this would involve considerable addi-
tional resources and effort. , ...
Tt w been suggested that:complex-cross populations derived
has genetically diverse collections should be mam-
‘iSS b “«Sof variability. An lot.rna.ional Gene
SV/bS'has been MM <•
rnat.riai°is made avaiUble to the depositors on requeit for selection
of desirable types.
„ c-nftiiaries It has been proposed that within the centres of
Gene Sanctuaries « diversity should be demarcated and pro-
origin areas of the groa ^ ", such areas, the evolutionary
tented. from ?“““ “"S. «t>4 the enSnment would be
potential of the to0JJ| P 1 presetve vari.bilitj in these popu-
preserved. This worn „ ; o f ution t0 continue and create new
lat,0nS WR U p t r ^ ^‘looses to' SJhpne sanctuaries in Meghalaya
forces and in the North-Eastern Region for Musa, Citrus, Oryza ,
Sacchamm and Mangijera.
Thus a eene sanctuary may be defined as an area of diversity
, interference from man. A gene sanctuary con-
protecied agamst J £ environment where it naturally
"S2 VWnoSnl ”™e“Ae gern.pl.sm with very little labour
exoer.se" but also permits. evolution to proceed on its natural
course S allows the appearance of new gene combinations and
new alleles not present in the preexisting population.
EXPLORATION
Trvnlnrntians are trips for the purpose of collection of various
forms of crop plants and their related species. Explorations generally
i°' m p s r fw areas that are likely to show the greatest diversity of
Tt, e centres of origin ate such areas and are often visited by
Wfous IxplSStion teams. In addition, the land i vanettes and
‘ Pollinated varieties are also collected. Exploration is the
witnary source of ail the germplasm maintained in germplasm
collections.
An expedition has to be meticulously planned. It must be
based on a sound knowledge of the biosystematics of the genera and
sneries to be collected. The ecology of the vegetation types and the
climatology of the region should also be considered. More emphasis
shS bf placed on ecological aspects. A preliminary survey may
Domestication, Plant Introduction and Acclimatisation
33
be conducted before the collections are actually made. The job of
^ *§l an exploration is a difficult one : as many diverse types as possible
y-f should be collected \ and , as far as possible , duplication in the
, • collections ^hould be avoided. Generally, it is extremely difficult to
avoid duplications: this, in turn, leads to overcrowding of germplasm
collections. , This becomes more difficult because the "value of
various types, either directly as varieties or as parents in hybridiza-
tion, cannot be accurately predicted from their appearance, i.e.,
phenotype.
Indian scientists have undertaken several explorations. In
1955, the Botanical Survey of India sent a team to Bonidila, NEFA.
•In 1961, an expedition to Central Nepal between Buuval and Pokhra
and Muktinath was undertaken. Collections made during this
expedition included cultivars of cereals, millets, pulses, mustards,
wild species related to wheat, oat, barley, linseed okra and several
other crops. Dr. K. B. Singh toured the country extensively
in connection with germplasm collection. In addition, explorations
were organised in tribal areas of Bihar, Orissa and Andhra Pradesh,
In Lahaul and Spiti, in rainfed areas of Madhya Pradesh, Rajasthan
and Gujarat, in the North-East Region, in Indonesia and certain
parts of U.S.S.R. At present scientists in various coordinated
projects of ICAR are required to make collections of land varieties
and forward them to the concerned central institutes for evaluation
and maintenance.
PURPOSE OF PLANT INTRODUCTION
The main purpose of plant introduction is to improve the plant
wealth of the country. The chief objectives of plant introduction
may be grouped as follows.
1 To Obtain an Entirely New Crop Plant. Plant introductions may
provide an entirely new crop species. Many of our improiant crops,
e.g maize, potato, tomato, tobacco, soybean etc., are introductions.
To Serve as New Varieties. Sometimes, introductions are released
as superior commercial varieties. The Mexican semidwarf wheat
varieties, Sonora 64 and Lerma Rojo, semidrawf rice varieties,
p TN 1 and IR 8, are more recent examples of this type.
To be used in Crop Improvement. Often the introduced material
is used in hybridization with local varieties to develop improved
varieties. Pusa Ruby tomato was derived from a cross between
ip Meeruty and Sioux, an introduction from U.S.A.
To Save the Crop from Diseases and Pests. Sometimes a crop is
introduced into a new area to protect it from diseases and pests.
Coffee was introduced in South America from Africa to prevent
losses from leaf rust, ffevea rubber, on the other hand, was
1 brought to Malaya from South America to protect it from a leaf
disease.
New Crop Species. The crops introduced in India include such
important crops as potato, maize, groundnut, chillies, coffee
Hevea rubber, guava, grape, pineapple, papaya etc. Several orna-
like gulmohar, phlox, salvia, aster etc. are introductions
Plmt Breeding : Principles and Methods
For Scientific Studies. Collections of plants have been used
in studies on biosystematics, evolutionand origin of plant species.
NX Vavilov developed the concept of centres of origin and that
of homologous series in variation from the study of a vast collection
of plant types.
Aesthetic Value. Ornamentals, shrubs and lawn grasses are
introduced to satisfy the finer sensibilities of man. These plants are
used for decoration and are of great value in social life.
SOME IMPORTANT ACHIEVEMENTS OF PLAN1
INTRODUCTION
The extensive movements of plants from their centres of origin
are primarily due to introductions in the prehistoric times. Almost
all the countries in the world have obtained some entirely new crops
through introduction. In India, introduced materials have been
used directly as varieties, released as varieties after selection or used
in hybridization programmes. The examples of varieties developed
from primary or secondary introductions are too numerous to be
listed in toto. Some important examples will be cited to illustrate the
achievements of plant introduction.
35
Domestication, Plant Introduction and Acclimatisation
Some important recent introductions are : soybean and sugarbeet
(in 1960). Introduction pf oil palm and ojoba is still in the experi-
mental stages.
Directly Released as Varieties. Semidwarf wheat (F. aestivum)
varieties, Sonora 64 and Lerma Rojo, were released directly for
cultivation. TN I rice (O. sativa) was introduced from Taiwan and
directly released as a variety. Other introduced rice varieties are IR
8, IR 21 and IR 28 from IRRI, Philippines. Other examples include
Ridley wheat and Kent oat (A. sativa ), both from Australia ;
Bonneville and Early Badger peas (P. sativum) ; Delcrest and
Virginia Gold tobacco (N. tabacum) ; Bragg, Lee, Clark 63 and
Hill Davis soybeans (G. max) ; and Sioux tomato (L. esculentum ),
all from USA. Several Introductions of vegetable cowpea,
cauliflower, onion, lettuce, watermelon etc. have been released as
varieties.
Varieties Selected from Introductions. Many varieties have been
developed by selection from introductions. Two varieties of wheat,
Kalyan Sona and Sonalika, were selected from introductions from
CIMMYT, Mexico. These varieties dominated wheat cultivation in
India for about one decade. Other varieties developed through
selection are Jamnagar Giant and Improved Ghana bajra ; Pusa
Lai and Pusa Sunehari sweet potato, Pusa Basmati, vegetable
cowpea, and Japanese White and 40 Days radish etc.
Varieties Developed through Hybridization. Introductions have
contributed immensely to the development of crop varieties through
hybridization. AH the semidwarf wheat varieties are derived from
crosses with Mexican semidwarf wheats. All but few semidwarf
rice varieties possess the dwarfing gene from Dee-geo- woo- gen,
either from TN 1 or IR 8. Thus almost all the semidwarf wheat and
rice varieties have been developed from crosses involving imroduc- '
tions. The hybrid maize, jowar (5. bicolor) and bajra (P.
americanum ) varieties generally have one parent, the male sterile (or
the female) parent, which is either an introduction or derived from
an introduction. AH the sugarcane vaneties have been derived
from introduced noble canes, Saccharum cfficinarum.
Other examples of varieties developed through hybridization
with introductions are Pusa Ruby tomato obtained from a cross
between Meeruti and Sioux ; Pusa Early Dwarf Tomato derived
from the cross Meeruti X Red Cloud ; Pusa Kesar carrot, Pusa
Kanchan turnip, AF 3 and S 350 bajra etc.
MERITS OF PLANT INTRODUCTION
1. It provides entirely new crop plants.
2. May provide superior varieties directly, after selection or
hybridization.
3. Introduction and exploration are the only feasible means of
collecting germplasm and to protect variability from genetic
erosion.
Plant Breeding : Principles and Methods
4. It is a very quick and economical method of crop improvement,
particularly when the introductions are released as varieties
directly or after simple selection.
5. Plants may be introduced in new disease-free areas to protect
them from damage, e.g., coffee and rubber.
DEMERITS OF PLANT INTRODUCTION
The disadvantages of plant introduction are associated with
the introduction of weeds, diseases and pests In the past, several
such cases have occurred, some of which are listed below.
Weeds. Argemone mexicana. Eichornia crassipes and Phylaris
minor (gehunsa) are some of the noxious weeds introduced in India.
There are many other instances of weeds that were introduced from
other countries. .
Diseases. Late blight of potato was inroduced from Europe in
1883. Flag smut of wheat was introduced from Australia ; coffee
rust came from Ceylon in 1876 ; bunchy top of banana was intro-
duced from Ceylon in 1940. Some other diseases were also-
introduced in India aiongwith plant materials.
Insect Pests. The potato tuber 'moth came from Italy in 1900.
Woolly aphis of apple and fluted scale of Citrus were also introduced
in India aiongwith plant introductions.
tome introduced species may become noxious weeds in the new
habitat. Water hyacinth and Lantana camara (kuri) were both intro-
duced as ornamental plants, but they are now noxious weeds.
Some introduced species may disturb the ecological balance in
their new home, and may cause serious damage to the ecosystem
Eucalyptus sp. introduced from Australia cause a rapid depletion of
the subsoil water reserves. Many scientists fear that lame scale
planting of Eucalyptus in India may deplete the subsoil and 'under-
ground water reserves, which is bound to be detrimental to the
ecosystem.
However, most of the cases of introduction of weeds, diseases
and insect pests occurred during a period when quarantine was
almost nonexistent. At present, plant introductions have to satisfy
rigid quarantine laws. They are thoroughly examined for weeds and
are fumigated for diseases and pests before their entry is permitted
Thus; at present there is little chance, if any, for the entry of weeds*
diseases and pests with plant introductions. But this underlines the
necessity for vigilance by the officials responsible for the enforcement
of quarantine laws. An error, lapse or carelessness on their part mav
allow entry to a serious disease, pest or weed species This mav
cause considerable economic losses to the nation. 1
Domestication, Plant Introduction amd Acclimatisation 37
SUMMARY
Crop plants originated from wild species through Mendcllao variation,
interspecific hybridization or polyploidy. In all these modes of evolution,
natural and, perhaps* artihcia? selections played an important role. The crops
and their related wild species show greater diversity in certain areas. Vavilov
proposed that these centres of diversity are the centres of origin of thfc crop
species. There are eight main centres of origin, viz., China, Hindustan,
Central Asia, Asia Minor. Mediterranean, Abyssinia, Central America and
South America. It may be more appropriate to call these as centres of
diversity rather than centres of origin.
Plant introduction consists of taking a plant species or variety into a
new environment where it has not been grown before. Plant movement
accompanied the movement of man. In the past, plant introduction was done
by invaders, settlers, travellers, traders, pilgrims and explorers. India received
a number of, plant species through early introductions. At present, plant
Introduction is the responsibility of the National Bureau of Plant Genetic
Resources situated in I ARC, New Delhi. The bureau has four substations at
Simla, Jodhpur, Kaoya Kumari and Akola for evaluation and maintenance
of plant collections. The functions of the bureau include introduction,
evaluation, multiplication and maintenance of germpiasm. Quarantine of the
introduced plant materials is also done by the bureau. The bureau supplies
germpiasm to scientists on request.
The germpiasm to be introduced is obtained from other similar agencies
in foreign countries or is collected through exploration. The introduced
material must be free from weeds, diseases and insect pests. It is the responsi-
bility of the NBPGR to ensure that the introduced material conforms to
quarantine regulations. The introduced material must be accompanied by an
authentic phytosanitary certificate from the source country.
The introduced material is catalogued, evaluated and, if found suitable,
multiplied and released directly as a new variety. Sometimes, new varieties are
developed by selection from the introduced material. Performance of some
introductions may improve through acclimatisation. Acclimatisation is a
process in which genotypes adapted to the new environment increase, while
those poorly adapted decrease in proportion. Thus acclimatisation is a pro-
cess of natural selection More often, introductions are used as parents in
hybridization programmes. Several important crop varieties have resulted
from plant introduction.
Germpiasm represents the sum total of the genes present in a species.
Germpiasm collections are maintained to preserve the genetic variability in
crop species and their wild relatives. This is becoming increasingly important
due to the ever incre ising genetic erosion caused by human activities. Due to
genetic erosion, wild relatives of crops and the variability present in the crop
species is being depleted.
QUESTIONS
What are the differences between the following ? (i) Domestication and
introduction, (ii) Natural and artihcial selection, (iii) Gene bank and
gene sanctuary, (iv) Centres of origin and centres of diversity, (v) Selec-
tion and acclimatisation, (vi) Primary and secondary introductions, and
(vii) Introduction and in regression.
Define domestication. What are the important changes that have occurred
under domestication ?
3 ,
Describe with the help of suitable examples the sources of variation that
have led to the evolution of crop plants.
4 .
(Jive, the chief contributions of the following scientists ; (i) NX Vavilov*
and (ii) H.B. Singh.
P
1
38
Flam Breeding : Principles and Methods
5. List the various centres of origin of cultivated plants. Briefly discuss the
concept of centres of oiigin (primary and secondary), la what way is this
concept helpful in plant breeding ?
6. Describe in brief the purpose of plant introduction and outline the various
steps involved in plant introduction.
7. What do you understand from quarantine ? Why is it necessary to
quarantine plant introductions ? Discuss in detail giving suitable
examples.
8. Is genetic variability within a variety essential for : (i) Acclimatisation,
(ii) Selection, (iii) Introduction, and (iv) Domestication ? Why ?
9. ' Desciibe some important achievements of plant introduction. Discuss
the merits and demerits of plant introduction as a breeding method.
10. India has acquired a large plant wealth through plant introductions, and
the increased agricultural productivity in the recent years can be traced
to some critical introductions. Discuss in detail with the help of suitaole
examples.
1L Write short notes on the following : (i) Introgressive hybridization,
(ii) Centres of origin of crop plants, (iii) Acclimatisation, (iv) Germplasm
collection, (v) Exploration, (vi) Microcentres, and (vii) Primary introduc-
tion.
12. What is NBPGR ? List the objectives of NBPGR and describe its orga-
nisation and functions.
Suggested Further Reading
Allard, R.W. 1960. Principles of Plant Breeding. John Wiley and Sons. Inc.,
New York.
Elliot, F.C. 1958. Plant Breeding and Cytogenetics. McGraw-Hill Book Co.,
Inc , New York.
Frankel, O.H. and Bennet, E. (eds.) 1970 Genetic Resources in Plants.
Their Exploitation and Conservation. Blackwell, Oxford (International
Biological Programme, Handbook II).
Frankel, O H. and Hawkes, J.G (eds.). 1975. Crop Genetic Resources for
Today and Tomorrow. Cambridge University Press (International
Biological Programme, 2).
Harlan, J.R. 1951. Anatomy of gene centres. Anaer. Nat. 85 : 97-103
Harlan, J R. 1956. Distribution and utilization of natural variability in culti-
vated plants. Brookhavea Symp. Biol. 9 : 191*208.
Harlan, J.R. 1971. Agricultural origins : centres and noncentres. Science 174 :
468474.
Harlan, J.R. 1975. Geographic patterns of variation in some cultivated plants.
J. Heredity 66 : 184-191.
Hutchinson. J.B. (ed.). 1974. Evolutionary’ Studies in World Crops.
Cambridge University Press.
IBPGR. 1975. The Conservation of Plant Genetic Resources. IBPGR, Rome.
Li. H.L. 1970. The origin of cultivated plants in Southeast Asia. Econ. Bot.
24 : 3-19.
Mangelsdorf, P. C. J 952. Evolution under Domestication. A me r. Nat. 86 :
65-77.
Pickersgili, B. 1977. Taxonomy and the origin and evolution of cultivated
plants in the new world. Nature, Lond. 268 : 591-595.
Domesttcation, Plant Introduction and Acclimatisation 39
Council of Agricultral
Cuitivated Pi — Harvard University
SlM TwYork W ‘ (ed ‘ ) - J976 ‘ Ev0,ut ' ;on of Cr °P **•««. Longman.. London and
Sl Tnd D NewYork 79 ’ PrincipIes of Cr °P Improvement. Longman. London
Singh. H.B. and Hardas. M.W. 1970. Plant Introduction. In New Vistas in
Grop Yields-Agricuiture Yearbook, ICAR. New Delhi, pp. 8-32.
Smith, C.E. 1969. From Vavilov to the present-a review. Econ Bot. 23 : 2-19.
Vav ilov N I. 1 9 51 The Origin, Variation. Immunity and Breeding of Culti-
CHAPTER 3
Modes of Reproduction and
Pollination Control
The mode of reproduction determines the genetic constitution
of crop plants, that is, whether the plants are normally homozygous
or heterozygous. This, in turn, determines the goal of the breeding
programme. If the crop plants are naturally homozygous, e.g.,
as in self- pollinators like wheat, a homozygous line would be desir-
able as a variety. But if the plants are heterozygous naturally, e g
as in cross-pollinators like maize, a heterozygous population has to
be developed as a variety. Consequently, the breeding methods have
to be vastly different for the two groups of crop plants The
knowledge of the mode of reproduction of crop plants is also
important for making artificial hybrids. Production of hybrids between
diverse and desirable parents is the basis for almost all the modern
breeding programmes.
MODES OF REPRODUCTION
The mode of reproduction in crop plants may be broadlv
grouped into two categories, asexual and sexual.
Asexual Reproduction
Asexual reproduction does not involve fusion of male and
female gametes. New plants may develop from vegetative parts of
the piant (vegetative reproduction) or may arise from embryos that
develop without fertilization (apomixis).
Vegetative Reproduction. In nature, a new plant develops from a
portion of the plant body. This may occur through modified under-
ground and sub-aerial stems, and through bulbils
Modes of Reproduction and FoUimtwn, Control
Tuber Potato (Solatium tuberosum)
Bulb-Onion (Allium cepa), garlic (Allium sativum )
Rhizome Ginger (Zinger office), turmeric (CmmuJomaM)
<Cto -—A — (Colocasia escu ,J a)
sucker etc. ^ub-aeria^sm^are^lmed 8 f aciude runncr . stoioa,
{Mentha sp.), date piam {Phoenix liactiU/era) of mint
„ B ilSt SJXS2J5TZ2? d ™'°> >«° pi—
tbeir development does not iili l r ;£• . are vegetative bodies -
The lower flowers in the inflorescencVofT °r 3nd seed f °rmation.’
;■ h “ b “”
and gewee are in commoifusefofn CtC ' L . ayerin Z> budding,
and ornamental shrubs St! f Propagation of fruit treS
multiplication through tissue Culture in f °f ve S etat ive
and attempts are being madp rn!ti ° Ca l of ma ?y plant species,
other plant species. In manv of thp«?°? tbe tecbn i<31&s for many
occurs naturally but for certain reason- ^ eC5es “ xuaI ' reproduction
more desirable. n reasoni vegetative reproduction is
may be used as a variety direct \ desirable plant
gous or heterozygous Further' ^ ar dless of whether it is faomozy-
if desirable, ean be mulifplied
tS! x %Z^T d S" ,h ‘ r*™ *«**
are identical in genotvre t,y ’ th P lams resulting from them
sexual reproduction is either suonf^f */ lant '. In a Pomictic species,
reproduction does occur the anomtv* ^, cr abseDt - When sexual
when sexual reproduction is abserf itV* ^ r “ e . d as Mutative. But
crop species show apomixis £ referred f a ! obBgatt. Many
details of apomictic reDrod..A,v, * f 8 ene rally facultative. The
terminology has resulted A s nolified Ho wdeJy . that / confusing
given below. * simplified classification of apomixis is
Advert five Embryonv Tn ■■■• : /■■■■■.
from vegetative cells of thi'l C f Se ’ ? mbr y° s develop directly
and chalaza. SevdopSent J 'S ^ as miceI3us > integument!
of embryo sac. Adventive emhrf ^^ 0 does a ? £ Jnvolve production
indica). Citrus, e£ tmhxyony occurs in mango (Mmgifera
42
Plant Breeding : Principles and Methods
Apospory. Some vegetative cells of the ovule develop into
unreduced embryo sacs after meiosxs. The embryo may develop
from egg cell or some other cell of this embryo sac. Apospory
occurs in some species of Hieruceum, Mai us, Crepis, Ranunculus?
etc.
Diplosporv . Embryo sac is produced from the megaspore, which
may be hapliod or, more generally, diploid Generally the meiosis is
so modified that the megaspore remains diploid. Diplospory leads to
parthenogenesis or apogamy .
1, Parthenogenesis. The embryo develops from
Depending upon whether the embryo sac is haploid or diploid,,
parthenogenesis is termed as haploid or diploid parthenogenesis \
Haploid parthenogenesis occurs accidentally and has been reported
in Salanum nigrum , Nicotiana , Crepis and maize. Diploid partheno-
genesis occurs in many grasses, e.g . Taraxacum .
In many species, e.g., Nicotiana , Datura , rice etc., pollen grains
may be induced in vitro to' produce haploid embryos or plantlets.
For this, anthers or pollen grains are cultured on a suitable tissue
culture medium. This technique offers several possibilities for crop
improvement.
2. Apogamy. In apogamy , synergids or antipodal cells develop into
an embryo. Like parthenogenesis, apogamy may be haploid or diploid
depending upon the haploid or diploid state of the embryo sac.
Diploid apogamy occurs in Antennaria , Alchemilla , Allium and many
other plant species.
Significance of Apomixis. Apomixis is a nuisance when the breeder
desires to obtain sexual progeny, i.e.> seifs or hybrids. But it is of
great help when the breeder desires to maintain varieties. Thus in
breeding of apomictic species, the breeder has to avoid apomictic
progeny when he is making crosses or producing inbred lines. But
once a desirable genotype has been selected, it can be multiplied and
maintained through apomictic progeny. This would keep the
genotype of the variety intact. Asexually reproducing crop species
are highly heterozygous and show severe inbreeding depression.
Therefore, breeding methods in such species must avoid inbreeding.
Sexual Reproduction
Sexual reproduction involves fusion of male and female gametes
to form a zygote which develops into an embryo . In crop plants,
male and female gametes are produced in specialised structure
known as Sowers.
Flower. A flower usually consists of sepals, petals (or their modifica-
tions), stamens and/or pistil. A flower containing both stamens
and pistil is a perfect of hermaphrodite flowers. If it contains
stamens but not pistil, it is known as staminate while a pistillate
flower contains pistil but not stamens. Staminate and pistillate
flowers occur on the same plant in a monoecious species, such as,.
L LJR™
BhMMK
jHv’Sg
m tiimmiMm
KM
Mooes of Reproduction and Pollination Control
“ aize > Colocasia, castor {Ricinus communis), cococut e
dioecious species, staounate and pistillate flowers occur o
PmZZ/f’ W ya ’ d 5 e P alm (^emjc dactylifera),
(Pistaaa vera), hemp (Cannabis indica), etc. The mal
filament
OU£n
*' ’OTHER
CELLS
fPMC*®;
transverse section of A
YOUNG ANTHER
maturation
meiotic
DIVISION
MEIOTIC
DIVISION
POLLEN GRA.W
(MICROSPORE;*
QYAO
MICROS PO« OGENES1S
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MITOSIS
►PERMS
VEGETATIVE/
TUBE NUCLEUS
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POLLEN
TUBE
NUCLEUS
44
Pbrnt Breeding : mi Methods
Microsporogenesis. Each anther has four poSteai sacs which
contain numerous pollen mother cells (PMCs). Each PMC under*
goes meiosis to produce four haploid cells or microspores. This
process is known as microsporogenesis (Fig. 3.1). The- miorospores
mature into pollen grains by thickening of their walls.
Megasporogenesis. Megasporogenesis occurs in ovules which are
present inside ovary. A single cell in each ovule differentiates into
a megaspore mother cell The megaspore mother, cell undergoes
meiosis to produce four haploid megaspores. Three of the megaspores
degenerate leaving one functional megaspore per ovule (Fig. 3.2).
7
Garnet ogenesis. The production of male and famale gametes in the
microspores and megaspores is known as gametogenesis.
Microgamelogenesis. This refers to the production of male gamete
or sperm. During the maturation of pollen, the microspore nucleus
divides mitotically to produce a generative and a vegetative or tube
nucleus . The pollen is generally released in this binucleate stage .
When the pollen reaches stigma of a flower, it is known as pollina-
tion. Shortly after pollination, the pollen germinates. The pollen
tube enters the stigma and grows through the style. The generative
nucleus now undergoes a mitotic division to produce two male
gametes or sperms. The pollen, alongwith the pollen tube, is known
as microgameiophyie (Fig. 3.1). The pollen tube finally eaters the
ovule through a small pord, micr.opyle , and discharges the two
sperms into the embryo sac.
M eg agametogenesis. The nucleus of a functional megaspore
djvides mitotically to produce four or more nuclei. The exact
number of nuclei and their arrangement varies considerably from
one species to another. In most of the crop plants, megaspore nucleus
undergoes three mitotic divisions to produce eight nuclei. Three of
these nuclei move to one pole -and produce a central egg ceil and two
synergid cells . One synergid is situated on either side of the egg cell.
Aoorher three nuclei migrate to the opposite pole to give rise to
antipodal cells . The two nuclei remaining in the centre, the polar
nuclei, fuse to form the secondary nucleus . The megaspore thus
develops into a mature megagametophyte or embryo sac. The
development of embryo sac from a megaspore is known as megaga-
metogenesis. The embryo sac generally contains one egg cell, two
synergids, three antipodal cells (all haploid), \ and one diploid
secondary nucleus (Fig* 3.2).
Fertilization. The fusion of one of the two sperms with the egg cell,
producing a diploid zygote is known as fertilization . The fusion of
the remaining sperm with the secondary nucleus leading to the for-
mation of a triphid primary endosperm nucleus k termed «t triple
fusion. The zygote divides jnitoiicaily to produce a diploid embryo,
i oe primary endosperm nucleus produces endosperm through
Modes of-Reprodmim and PoiKnuMm Control
X
fVtSGASPORf
MOTHER CEU
'•ONG.ruo.NAt SECT, ON qf
ovule
FIRST (VJEiOTtC
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MEIOTIC
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MITOSIS
MEGASPORP
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anUpooal CELLS
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MSGAOAMCTOPM YTE ■
ISMaRYOSACi
p. » ' ' ISWaRYOSACs
46
Plant Breeding : Principles an d Methods
Significance of Sexual Reproduction. Sexual reproduction makes it
possible to combine genes from two parents into a single hybrid
plant. Recombination of these genes produces a large number of
genotypes. This is an essential step in creating variation through
hybridization. Almost the entire plant breeding is based on sexual
reproduction. Even in asexualiy reproducing species, sexual repro-
duction, if it occurs, is used to advantage, e.g., sugarcane, potato,
sweet patato etc.
ANTHESIS
In the process of flowering , the first opening of a flower is
known as anthesis. The details of anthesis vary from one crop
species to another. They are also greatly affected by environmental
factors, such as humidity and temperature. A knowledge of the anthe-
sis of a crop is desirable for making successful crosses, and also it
largely determines the mode of pollination prevalent in the crop. In
case of rice (O. sativa), the flower is enclosed by two /boat- shaped
bracts, lemma and palea. Petals and sepals are represented by two
small sac like lodicules . It has six stamens and one ovary. The
ovary has two elongated and feathery stigmas. When stigmas
become receptive, the lodicules gradually swell and push the lemma
and palea apart. Simultaneously, the filaments of stamens
elongate pushing the anthers out of the bracts. The anthers dehisce
during this process and liberate most of their pollen inside the flower.
The remaining pollen grains fall outside./ On reaching the stigma,
the pollen germinates within few minutes. The pollen tube reaches
the embryo sac in about 20 minutes to two hours. Sometimes the
anthers may be pushed out before they dehisce, or the pollen may
not be functional. In such cases, cross-pollination may take place.
In rice, as high as 2-3 per cent cross-pollination may occur.
MODES OF POLLINATION
Pollination refers to the transfer of pollen grains from anthers to
stigmas. Pollen from an anther may fall on stigma of the same
flower leading to self-pollination or autogamy . When pollen from
flowers of one plant are transmitted to stigmas of flowers of another
plant, it is known as cross-pollination or allogamy . A third situation,
geitonogamy, results when pollen from a flower of one plant falls on
the stigmas' of other flowers of the same plant, e.g. 9 in maize
(Z. mays). The genetic consequences of geiionogamy are the same
as those of autogamy.
Self-pollination
Many cultivated plant species reproduce by self-pollination
{Table 3. 1). Self-pollinated species are believed to have originated
from cross-pollinated ancestors. These species, as a rule, must have
hermaphrodite flowers. But in most of these species, self-pollination
Modes of Reproduction and Pollination Control
47
is not complete and cross-pollination may occur up to 5%. The
degree of cross-pollination in self-pollinated species is affected by
several factors, e.g ., variety, environmental conditions like tempera-
ture and humidity, and location. There are various mechanisms that
promote self-pollination, which are generally more efficient than
those promoting cross-pollination. These machanisms are listed
below.
1. Cleistogamy . In this case, flowers do not open at all. This ensures
complete self-pollination since foreign pollen cannot reach the
stigma of a closed Sower. Cleistogamy occurs in some varieties
of wheats ( Triticum sp.), oats { Avena sp.), barley ( H . vidgare)
and in a number of other grasses.
2. In some species, the flowers open (chasmogamy), but only after the
pollination has taken place. This occurs in many cereals, such
as, wheat, barley, rice and oats. Since the flower does open,
some cross-pollination may occur,
3. In crops like tomato (I. esculentum ) and brinjal ($. melongena ),
the stigmas are closely surrounded by anthers. Pollination gene-
rally occurs after the flowers open. But the position of anthers
in relation to stigmas ensures self-pollination.
4. In some species, flowers open but the stamens and the stigma
are hidden by other floral organs. In several legumes, e.g., pea
(P. sativum), mung (V. radiata), urd ( V . mungo), soybean (G.
max ) and gram ( C . arietinum), the stamens’ and the stigma are
enclosed by the two petals forming keel.
5. In a few species, stimgas become receptive and elongate through
stamina! columns. This ensures predominant self-pollination.
Genetic Consequeness of Self Pollination. Self-pollination leads, to a
very rapid increase in homozygosity. Therefore, populations of self-
pollinated species are highly homozygous. Self-pollinated species do
not show inbreeding depression, but may exhibit considerable
heterosis. Therefore, the aim of breeding methods generally is to
develop homozygous varieties.
Table. 3.1. Classification of crop species on the basis of their natural mode
of pollination
SELF-POLLINATED CROP SPECIES
Cereals and Millets
Wheat ( Triticum aestivum }
Rice (Or) za sariva )
Barley (Hordeum vulgare)
Oats (Avena sativa)
Foxtail millet (Setaria ttalica )
Ragi (Eleusine coracana)
Legumes
Pea (Pisim sativum)
•Groundnut ( Arachis hypogaea )
Gram (Cicer ariatinum)
Mung ( Vigna radiata)
Urd ( Vigna mango)
Cowpea (Vigna onguiculata)
Soybean ( Glycine max)
Sem ( Doiichos lablab)
Lentil (Lens escuhnta)
Khesari ( Lathy r us sativus)
Rajma { Phaseofus vulgaris )
Guar (Cyamopsis tetragonoloba)
Moth (Phoseolus aconitifolius)
Sunnhemp {Crotalarld juncea)
Plant Breeding t Principles and Methods
Carrot (Daucut carotd)
Cauliflower (Brassica ok raced)
Cucumber (Cucumis sativus)
Onion {Allium cepa)
Pumpkin (Curcurbita maxima)
Radish (Rapkanus sativus)
Turnip (Brassica rapa >
Muskmelon ( Cucurbita moschata)
Watermelon ( CUrullus vulgaris)
Squash (< Cucurbita melanosperma)
Sweet potato (tpomoea batatas)
Others cucurbits (Cucurbita sp.)
Beets ( Beta vulgaris)
Broccoli (Brassica ole raced)
Brussels sprouts (Brassica oleracea)
Parsley (Petroselinum hortense)
Celery (Apium graven lens)
Spinach (Spinacea oleracea)
Asparagus (Asparagus officinalis)
Garlic (Allium sativum )
Coriander (Coriandrum sativum )
Fruits
Apple ( Pyrus maius)
Avocado {Per sea americana )
Mango ( Mangifera fndica )
Pear (Pyrus communis )
Blackberries ( Rubus fruticosus)
Raspberries (Rubus sp.)
Walnut C Juglans regia)
Chestnut (Costarica sativa and C
Vexa)
Hazelnut (Coryius americana and C
comma)
Banana { Musa sapient um)
Cherry (Prunus avium)
Date oa-lm ( Phoenix dactlUfera)
Fig (Ficus carica )
Coconut (Cocos nucifera)
Grapes (Vi i is vinifera)
Papaya ( Carica papaya )
Plum (Prunus divaricata)
Loquat (Eriobotrya Japan -ca)
Strawberries (Frag aria sp.)
Almond (Prunus amygdatus )
Pistachio nut (Pistacia vera)
OFTEN CROSS- POLLINATED
CROPS
Jo war (Sorghum bi color)
Cotton ( Gossypium sp.)
Broad bean (Viciafabd)
Jute (Cor chorus olitorius )
Tobacco ( Nicotiana tabacum and N
rustica)
Pigeon pea or arhar ( Cajanus cajan )
Rai ( Brassica juncea)
Brassica campestris var. yellow sarsom
(yellow sarson)
B. campestris var. toria {toria)
Safflower (Cartkamus tinctorius)
Triticale (Triticaie hexcmloid
Other Crops
Jute (Cor chorus capsularis )
Vegetables
Tomato ( Lycoperstcon esculentum)
Okra ( Abeimoschus esculentus)
Lettuce ( Laciuca sativa )
Brmjal (Solanurn mdongena)
Chillies (Capsicum annuum)
Parsnip (Pastinaca sativa)
Potato (Saianum tuberosum )
Forage Crops
Burr clover (Medicago hispida)
Subterranean clover (Trifolium sub-
terraneum)
Velvet bean ( Mucuna deeringiana)
Slender wheatgrass (Agropyron paucb
fiorum )
Several other grasses
Oilseeds
Til (Sesamum indicum )
Linseed (Linurn usitatissimum)
Fruits Trees
Apricot (j Farms armeniaca)
Nectarine (Prunus p&rsica)
Citrus i Citrus sp.)
Peach ( Prunus persied)
CROSS-POLLINATED SPECIES
Cereals
Maize (Zea mays)
Rye (Secale cereale)
Bajra (Pennisetum americanum)
Oilseeds
Some strains of Brassica campestris
Sunflower ( HeUanthus annus)
Castor (Ricirns communis)
Niger (Guizotia abyssinica)
Legume®
Alfala (Medicago sativa)
Red clover ( Trifolium pratenss)
White clover (Trifolium repens)
Crimson clover (Tri folium ihcdrnatum )
Sweet clover (Melihtus officinalis)
Birdsfoot trefoil ( Lotus corniculatus)
Forage Crops
Ryegrass (Lolium perenne)
Timothy grass ( Phleum pratense)
Smooth bromegrass (Bromus inermis)
Johnson grass ( Sorghum halepense)
Other Crops
Sugarcane (Saccharum officinarum)
Some lines of potato (5, tuberosum)
Hemp (Cahabis indica )
Hops ( Hamulus lupulus)
Vegetable®
Cabbage ( Brassica oleracea)
Modes of Reproduction and Pollination Control
49
Cross-Pollination
, tV in cross-pollinating species, the transfer of pollen from a flower
^ to the stigmas of the others may be brought about by wind ( ammo -
: phily), water (hydrophily) or insects ( entomophily ). Many of the crop
plants are naturally cross-pollinated (Table 3.1). in many species, a
small amount (up to 5-10 per cent) of selling may occur. There are
several mechanisms that facilitate cross-pollination ; these mecha-
nisms are described briefly.
1. Diclfny. Dicliny or unisexuality is a condition in which the flowers
are either stamina te (male) or pistillate (female).
(a) Monoecy. Staminate and pistillate flowers occur in the same
plant, either in the same inflorescene, e.g. 9 castor, mango,
(Mangi/era indica ), banana ( Musa sapientum) and coconut, ox in
4 separate inflorescences, e.g, 9 maize. Other monoecious species are
cucurbits (Cucurbita sp.), walnut (Juglans regia), chestnut, straw-
berries (. Fragaria sp.,) robber (Hevea sp,), grapes (Vitis vinifera) and
cassava (M. utilissima ).
(b) Dioecy. The male and female flowers are present on different
plants,' the plants in such species are either male or female, e.g, 9
papaya (C. papaya ), date ( Phoenix dactilifera ), hemp ( Cannabis
, indica),- asparagus ( Asparagus officinalis ), and spinach {Spinacea
oleracea). In general, the sex is governed by a single gene, e.g.,
asparagus and papaya. In some cases, there are hermaphrodite
plants in addition to males and females, and a number of inter-
mediate forms may also occur.
2. Dichogamy. Stamens and pistils of hermaphrodite flowers may
mature at different times facilitating cross-pollination.
(a) Protogyny. In crop species like bajra, pistils mature before
stamens.
(b) Protandry . In crops like maize and sugarbeets (B. vulgaris ),
stamens mature before pistils.
3. In lucerne or alfalfa (M. sativa), stigmas are covered with a
waxy film. The stigma does not become receptive until this waxy
film is broken. The waxy membrane is broken by the visit of honey
bees which also effect cross-pollination.
4. A combination of two or more of the above mechanisms may
occur in some species. This improves the efficiency of the system in
promoting cross-pollination. For example, maize exhibits both
' , monoecy and. protandry.
5. Self-Incompatibility. It refers to the failure of pollen from a
flower to fertilize the same flower or other flowers on the same plant.
Self-incompatibility is of two" types : sporophytic and gametophytic .
In both the cases, flowers do not sot seed on selfing. Self-incompa-
tibility is common in several species of Brassica (mustard,, rai,
cauliflower, etc.), some species of NLotiana , radish, rye and, many'
grasses.,; It is highly effective in preventing self-pollination. /
Plant Breeding : Principles and Methods
6. Male Sterility. Male sterility refers to the absence of functional
pollen grains in otherwise hermaphrodite flowers. Male sterility is not
common in natural populations. But it is of great value m experi-
mental populations, particularly in the production of hybrid seed.
Male sterility is of two types : genetic and cytoplasmic. Cytoplasmic
male sterility is termed cytoplasmic-genetic when restorer genes are
known. In view of the importance of self-incompatibility and male
sterility, a more detailed discussion on them follows later.
Genetic Cohsequences of Cross-Pollination. Cross-pollination pre-
serves and promotes heterozygosity in a population. Cross-pollina-
ted species are highly heterozygous and show mild to severe
inbreeding depression and a considerable amount of heterosis. The
breeding methods in such species aim at improving the crop species
without reducing heterozygosity to an appreciable degree. Usually,
hybrid or synthetic varieties are the aim of breeder wherever the
seed production of such varieties is economically feasible.
Often Cross-Pollinated Species
In many crop plants (Table 3.1), cross-pollination often exceeds
5 per cent and may reach 30 per cent. Such species are generally
known as often cross-pollinated species, e.g., jo war, cotton (Gossy-
pium sp.), arhar ( Cajanus cajan ), safflower (C. tinctorius) etc. The
genetic architecture of such crops is intermediate between self-pollina-
ted and cross-pollinated species. Consequently, in such species
breeding methods suitable for both of them may be profitably applied.
But often hybrid varieties are superior to others.
DETERMINATION OF THE MODE OF REPRODUCTION
IN A SPECIES
The first step in determining the mode of reproduction of a
species is to critically examine its flowers. Mechanisms like dioecy,
monoecy, protogyny, protandry and cleistogamy are easily detected ;
they clearly indicate the mode of pollination.
The second step consists of isolating single plants and record-
ing seed set under isolation. Space isolation, i.e., individual plants
grown at sufficient distance to prevent cross-pollination, is preferable
to isolation by bags or cages. Isolation by bags or cages may create
an environment unfavourable for pollination and seed set. Failure
to set seed in isolation proves the species to be cross-pollinated.
However, setting of seeds is only indicative of self-pollination.
Finally , the effects of selling (inbreeding) on the vigour of
plants should be studied. Loss in vigour due to inbreeding is
common in cross-pollinators, but self-pollinators show no inbreed-
ing depression.
Modes of Reproduction and Pollination Control
51
DETERMINATION OF THE AMOUNT OF
CROSS-POLLINATION IN A SPECIES
The amount of cross-pollination is determined by planting two
strains of the species in a mixed stand. One strain is homozygous
for a dominant character, preferably, ah easily recognisable'seed or
seedling character, while the other strain has the recessive form of
the character. Each plant of the recessive strain is surrounded by
plants of the dominant strain to provide abundant pollen. The
seeds from the plants of recessive strain are harvested. The per-
centage of seeds carrying the dominant allele represents the
percentage of crosspollination in the species.
The frequency of cross-pollination varies greatly with the
variety, weather conditions and location. For example, in a study on
safflower, the estimates of outcrossing in different varieties grown in
the same year at the same location ranged from 0-8.7%. Similarly,
the amount of cross-pollination in a single variety grown at several
locations varied from 1.3 to 9.8 per cent. As a result, such a study
should include several varieties and locations, and should be conduc-
ted for two or more years. In view of this, the information on many
crops may be regarded as incomplete.
Whether a species is apomictic may be determined by crossing
a recessive strain (used as female) to a dominant one (used as male).
If a sufficiently high frequency of recessive offspring are recovered
in the progeny, the species is most likely apomictic. But every
precaution should be taken to avoid self-pollination while making
the crosses.
RELEVANCE OF THE MODE OF REPRODUCTION
TO PLANT BREEDING
The modes of reproduction and pollination are very important
in plant breeding because they determine (1) the genetic constitution
of a species, (2) the ease in pollination control, and (3) the stability
of varieties after release.
Genetic Constitution. The cross-pollinated species are highly
heterozygous, and generally show loss in vigour on inbreeding. The
self-pollinated species, on the other hand, consist of homozygous
individuals of similar or different genotypes. Self-pollinators do not
show inbreeding depression. The asexually reproducing crops are
similar to the cross-pollinated species. Primarily because of the
effects of inbreeding, the breeding approaches and the varieties
developed are different for the three different groups of crops. In
self-pollinating species, the varieties are homozygous, and often a
variety consists of plants of a single genotype. In the cross-pollinat-
ing species, on the other hand, the varieties are highly heterozygous
and consist of one (hybrid varieties) or many (open-pollinated and
synthetic varieties) genotypes. The asexually reproducing species
present an added advantage since asexual progeny from any
desirable plant at any stage of the breeding programme may be
52
Plant Breeding : Principles and Methods
directly used as a variety. In such crops, the varieties are clones, i.e.,
asexual progeny of single plants.
Facility in Controlled Pollination. Breeding methods invariably
depend upon some system of controlled mating. In most of the
breeding schemes, selected plants or strains have to be crossed.
The ease or the difficulty. in making these crosses and the precautions
necessary to ensure of- prevent selling largely depend upon the floral
structure and the mode of pollination of the species. For example, in
self-pollinating crops selfing occurs naturally, while m cross-
pollinating species flowers have to be hand-pollinated and protected
from foreign pollen.
Stability of Varieties after Release. Since in self-pollinators varieties
are homozygous and natural cross- pollination is negligible, they are
fairly stable in their genetic constitution. Farmers may plant the
same seed for several years if precaution is taken to avoid off- types
due to mechanical mixtures. Similar is the case with asexually repro-
ducing species. But in cross-pollinating species, sufficient precaution
has to be taken to avoid contamination by foreign pollen. In the
case of hybrid varieties, farmers have to replace the seed every year
due to severe inbreeding depression in the subsequent generations.
Even in case of synthetic varieties, the seed should be replaced every
few years. But asexually reproducing crops have a problem of their
own. They are often infected by one or the other virus which, unlike
sexually reproducing species, is transmitted from one generation to
the next. This leads to reduced vigour and yields, and necessitates
the replacement of seeds every few years.
MECHANISMS OF POLLINATION CONTROL IN
CROP PLANTS
There are several natural mechanisms that control the mode of
pollination in crop plants. These mechanisms have been briefly
described earlier while discussing self- and cross-pollination. Two of
these mechanisms, self-incompatibility and male sterility, are of
special significance because of theii utilization in hybrid seed produc-
tion. Therefore, these mechanisms will be considered in some detail.
Self-Incompatibility
More than 300 species belonging to 20 families of angiosperms
show self-incompatibility. Self-incompatible pollen grains fail to
germinate on the stigma. If some pollen grains do germinate, pollen
tubes fail to enter the stigma. In many species, the pollen tubes enter
the style, but they grow too slowly to effect fertilization before the
flower drops. Sometimes, fertilization is effected, but the embryo
degenerates at a very early stage. Self-incompatibility appears to'
be a biochemical reaction, but the precise nature of these reactions
is not clearly understood. The genetic control of incompatibility
reactions is relatively simple. Lewis (1954) has suggested various
classifications of self-incompatibility ; a relativelys imple classifica-
Modes of Reproduction and Pollination Control
53
tion is as follows : ( 1 ) heteromorphic system, ( 2 ) homomorphic
system, ( 2 a) gametophytic control, and ( 2 b) sporophytic control.
Heteromorphic System. In this system, flowers of different incompa-
tibility groups are different in morphology. For example, in Primula
there are two types of flowers, pin and thrum. Pin flowers have
long styles and short stamens, while thrum flowers have short
styles and long stamens. This situation is referred to as distyly.
Tristyly is known in some plant species, e.g„ Lythrum ; in such
cases, the style of a flower may be either short, long or of
medium length. In the case of distyly, the only compatible
mating is between pin and thrum flowers. /This characte-
ristic is governed by a single gene s, Ss producing thrum
and ss producing pin flowers.,; The incompatibility reaction
of pollen is determined by the genotype of the plant producing
them. Allele S is dominant over $ The incompatibility system, there-
fore, is heteromorphic-sporophyiic. ✓ Pollen grains produced by pin
flowers would all be s in genotype as well as in incompatibility re-
action. But pollen produced in thrum flowers would be of two
types genotypically, S and 5 , but all of them would be S pheno-
typicaily. The mating between pin and thrum plants would produce
Ss and ss progeny in equal frequencies (Fig. 3.3). This system is of
little importance in crop plants ; it occurs in sweet potato and buck-
wheat.
Homomorphic System. In the homomorphic system, incompatibility
is not associated with morphological differences among flowers.
The incompatibility reaction of pollen maybe controlled by the
genotype of the plant on which it is produced ( sporophytic control)
or by its own genotype (, gametophytic control).
Gametophytid System. Gametophytic incompatibility was first descri-
bed by East and Mangelsdorf in 1925 in Nicotian a sender ae. The
incompatibility reaction of pollen is determined by its own genotype,
and not' by the genotype of the plant on which it is produced (Fig. 3.4).
Generally, the incompatibility reaction is determined by a single
gene having multiple alleles, e.g., Trifolium, Nicotiana, Lycoperscion,
Solanum, Petunia etc. Sometimes, poiypioidy may lead to a loss of
incompatibility due to competition between the two S alleles in
diploid pollen. Irradiation of pollen or buds with X-rays or gamma-
rays temporarily suppresses the incompatibility reaction, and thus
allows the pollen tube to grow through incompatible style. In some
species, e.g , Phalaris, Physalis etc., two loci ( S and Z) govern
incompatibility, while in some others, e.g., Beta vulgaris and Papaver,
three loci are involved. In these cases, polyploidy does not affect the
incompatibility reaction. Pollen tube grows very slowly in the style
containing the same S allele as the pollen, and fails to effect fertili-
zation. Therefore, all the plants are heterozygous at the S locus.
In a single gene system, there are three types of matings :
'r ,
(i) Fully incompatible, e.g., S l S. 1 y.S l S 2
.
M ating
Progen y
Genoty pe
Phenotype
Genotype
Phenotype
Fin x pin
Pin x thrum
Thrum x pin
Thrum x thrum
ssx-ss
ssxSs
Ssxss
SsxSs
Incompatible Mating
1 Ss : 1 ss 1 Thrum
1 Ss : 1 ss . 1 Thrum
Incompatible Mating
Plant Breeding t Principles and Methods
\0J
PIN
.ss
G
ALL f
s
Phenotype of flower
Genotype of plant
(sporophyte)
Genotype of gametes
incompatibility
reaction of pollen
Incompatibility
y eaction of style
‘ e a
Modes of Reproduction and Pollination Control
genotype of plant
(SPOROPHYTE)
GENOTYPE OF
GAMETES
INCOMPATIBILITY
REACTION OF
POLLEN GRAINS
INCOMPATIBILITY
REACTION OF STY.LE
INCOMPATIBLE
COMPATIBLE
FULLY INCOMPATIBLE
MATING (SiS>xS,Si)
PARTIALLY COMPATIBLE FULLY COMPATlg
MATING (Si Sa xSi S3} MATING (Si Sj x Ss
Fig. 3.4. Gametorhytic system of incompatibility. The incompatibility reactic
of pollen is governed by the genotype of pollen itself. The incompai
bihty reaction of style is determined by its own genotype and general
the two alleles do not show dominance, that is, they show codominanc
The seif-incompatibility gene 5 has multiple alleles.
(ii) Fully compatible, e.g., S 1 S 2 XS 9 S i
(iii) Partially (t>., 50% of the pollen) compatible, e.g.,
02 X 020 3 .
In some cases, an allele for self-fertility, S/, is found.
Pollen carrying the Sr allele does not show incompatibility reaction
Thus tn a plant with the genotype S,S U selling produces S/S f and
progeny. Mutations for Sr allele may be induced by irradiating
the pollen used for self-pollination. There is another allele, Sf, which
ret ?uM S - th £2 rowth Sr pollen tubes, thus enforcing self-incom-
patibility. The gametophytic system is found in pineapple (2 locus)
ryegrass (2 locus), diploid coffee, diploid clovers (: Trifolium sp.) etc.
Sporophytic System. In the sporophytic system also, the self-
mc^paubihty is governed by a single gene, S, with multiple
alleles , more than 30 alleles are known in Brassica oleracea. In
general, the number of S alleles is considerably larger in the gameto-
phytic than m the sporophytic system. The incompatibility reaction of
Plant Breeding : Principles and Methods
GENOTYPE OP
PLANT
{SPOROPHYTE)
GENOTYPES OF
GAMETES
INCOMPATIBILITY
. REACTION OF
"POLLEN GRAINS
INCOMPATIBILITY
■REACTION OF STYLE
•POLLEN GENOTYPE
COMPLETE INCOMPATIBILITY COMPLETE INCOMPATIBILITY COMPLETE
(S*Sj SELFEO ) (Si S,’ x S i Sa} COMPATIBILITY
(Si Sax S a S3}'
The letters within parenthesis* e.g.,( S t ) and (Sa) denote the incompatibility
reactions of pollen grains and styles. A complete dominance Ssassumed.
Fig. 3.5. Sporophytic system of incompatibility. Incompatibility reaction of
pollen grains is controlled by the genotype of the plant (sporophyte)
on which they are produced, while that of style is governed by its
own genotype. For simplicity, it is assumed that the incompatibility
alleles (.S' alleles) show complete dominance in the manner Si>S t >
$i>St — etc. The actual situation is much more complicated and
several different types of allelic relationships are encountered.
pollen is governed by ike genotype of the plant on which the pollen is
produced , and not by the genotype of the pollen (Fig. 3.5). It was first
reported by Hughes and Babcock in 1950 in Crepis foetida , and by
Gerstel in Parthenium argent aturn (in the same year). In the sporo-
phytic system, the S alleles may exhibit dominance, individual action
(codominance) or competition. Consequently, there may be many
complex incompatibility relationships. Lewis has summarised the
following characteristics of this system :
1. There are frequent reciprocal differences.
2. Incompatibility can occur with the female parent.
3. A family can consist of three incompatibility groups.
4. Homozygotes are a normal part of the system.
5. , An incompatibility group may contain two genotypes.
Modes of Reproduction and Pollination Control 57
The sporophytic incompatibility m found is radish (J?.- sativus),
diploid Brassica crops and Sinapis . Is many cases, different S alleles
vary in their activity leading to varrying degrees of self-incompati-
bility, e.g*, B. oleracea. Polygenes (modifying genes) are known to
increase as well as decrease the activities of S alleles both iajhe
garaetophytic as well as sporophytic systems.
Mechanism of SelMncompatihllity
The mechanism of self-incompatibility is quite complex and is
poorly understood. The various phenomena observed in self-incom-
patible matings are grouped into three broad categories : (1) pollen-
stigma interaction,, (2) pollen tube-style interaction, and (3) pollen
tube-ovule interaction.
j Pollen-Stigma Interaction . These interactions occur just after the
pollen grains reach the stigma and generally prevent pollen germina-
tion. At the .time they reach stigma, pollen grains generally have
two nuclei in the garaetophytic system, while they have three nuclei
in the sporophytic system. This was once considered to be the basis
for the two incompatibility systems, but the available evidence indi-
cates otherwise. However, the structure of stigmatic surface appears
to be definitely Involved in the differences between two systems.
In the gametophytic system , the stigma surface is plumose having
elongated receptive cells and is commonly known as ‘wet* stigma.
The pollen grains generally germinate on reaching the stigma, and
the incompatibility reaction occurs at a later stage. There are clear-
cut serological differences among the pollen grains with different S
genotypes ; such differences have not been observed in the sporophy-
tic system.
In the sporophytic system, the stigma is papillate and dry, and is
covered with a hydrated layer of proteins known as ‘ pellicle*. There
is evidence that the' pellicle is involved in incompatibility reaction.
There are striking differences in the stigma antigens related to the
S allele composition. Within few minutes of reaching the stigmatic
surface, the pollen releases an exine exudate which is either protein
or glycoprotein in nature. This exudate induces immediate callose
formation in the papiSae (which are in direct contact with the pollen)
of incompatible stigma. Often, callose is also deposited on the
young protruding pollen, tubes preventing further germination of the
pollen. Thus in the sporophytic system, stigma is the -site of
incompatibility reaction ; once the pollen tube crosses the stigmatic
barrier, there is no further inhibition of the pollen tube growth. In
the homomorphic sporophytic system, the incompatibility reaction of
pollen is probably due to the deposition of some compounds from
anther tapetum on to the pollen' exine.
Pollen Tube-Style Interaction. In most cases of the gametophytic
system, pollen grains germinate and pollen tubes penetrate' the stig-
matic surface.' But in incompatible combinations, the growth of
■ pollen tubes is retarded within the stigma, e g., in Oenothera , or a little
later in the style, e.g., in Petunia, Lycopersicon, Ltlium etc. In the :
58
Plant breeding f Principles and Methods
latter case, there is a cessation of protein and polysaccharide
synthesis in the pollen tubes, which leads to the degeneration of
tube wall and the bursting of pollen tube.
Pollen Tube-Ovule Interaction . In some cases, e.g., Theobromo
cacao , pollen tubes reach the ovule and effect fertilization. However,
in incompatible combinations, embryos degenerate at an early stage
of development
Relevance of Self-Incompatibility in Plant Breeding
Self-incompatibility effectively prevents self-pollination. As a
result, it has a profound effect on breeding approaches and objectives.
These are discussed here in some detail.
1. In self-incompatible fruit trees, it is necessary to plant two
cross-compatible varieties to ensure fruitfulness. Further, cross-
pollinalion may be poor in adverse weather conditions reducing
fruit set. Therefore, it would be desirable to develop self-fertile
forms in such cases.
2. Some breeding schemes, e.g., development of hybrid varieties,
etc., initially require some degree of inbreeding. Although sib-
mating leads to inbreeding, but for the same degree of inbreeding
it takes twice as much time as selling. Further, for the
maintenance of inbred lines selling would be necessary.
3. Self-incompatibility may be used in hybrid seed production. For
this purpose, (1) two self-incompatible, but cross-compatible, lines
are interplanted ; seed obtained from both the lines would be
hybrid seed. (2) Alternatively, a self-incompatible line may be
interplanted with a self-compatible line. From this scheme, only
the seed from self incompatible line would be hybrid. (3) Schemes
for the production of double cross and triple cross hybridse faav
also been proposed and their feasibility has been demonstrated
in the case ofbrassicas.
The gametophytic system has been used, to a limited extent,
for hybrid seed production in clover. Trifolium (Leguminosae).
In Solanaceae, the cultivated species are generally self-fertile,
and self-incompatibility is confined to wild species. The
sporophytic system has been exploited for hybrid seed production in
brassicas (Cruciferae), primarily by the Japanese seed companies. In
Compositae, another economically important family showing sporo-
phytic self-incompatibility, the cultivated varieties are generally
self-fertile.
The use of self-incompatibility in hybrid seed production is
hampered by several problems. (I) Production and maintenance of
inbred lines by hand pollination is tedius and costly. (2) This raises
the cost of hybrid seed. (3) Continued selling leads to a depression
in self-incompatibility, and it unintentionally, but unavoidably,
selects for self-fertility. (4) In the gametophytic system, continued
inbreeding gives rise to new incompatibility reactions, which may
limit the usefulness of the inbreds as parents. (5) EvironmentaJ
Modes of Reproduction and Pollination Control
factors, e.g.y high temperature and high humidity etc., reduce or even
totally overcome self-incompatibility reaction, leading to a high
(30% or more) proportion of selfed seed. (6) Bees often prefer to
stay within a parental line, particularly when the parental lines
differ morphologically. This, in turn, increases the proportion of
selfed seed. (7) Transfer of S alleles from one variety or, more
particularly, species into another variety or species is tedius and
complicated. This has prevented the use of self-incompatibility in
hybrid seed production in Solanaceae and Compositae.
Elimination of Self-Incompatibility
In many cases, self-fertile forms will be highly desirable and, in
such cases, it would be useful to eliminate self-incompatibility. (IJfln
the case of single-locus gametophytic system, incompatibility may be
eliminated by doubling the chromosome nnmber, e.g., in potato
diploidizdtion leads to self-incompatibility. (2) Isolation of self-fertile
(St) mutations is a very useful tool in the elimination of self-incom-
patibility. Fldwer buds are generally irradiated at the PMC stage,
and pollen from these buds is used to pollinate flowers with known
S alleles/Generally, selection for Sr alleles is much more complicated
in the "sporophytic system than in the gametophytic system due to
the temporary loss in incompatibility and pseudofertility in the case
of the former. In Oenothera , St mutations occur spontaneously at
the rate of 10~ 8 , and the rate of induction with X-rays is 1.6 x 10” 8 /r
unit. Lastly, (3) self compatibility alleles may be transferred from
related species or varieties of the same species, if available, through
a backcross programme*.
Overcoming Self-Incompatibility
In many situations, e.g. f during the production of inbreds for
use as parents in hybrid seed production, it is essential that temporary
self-fertility is achieved in a manner so that self-incompatibility is
folly functional in the selfed progeny. Such self-fertility is known
as pseudofertility and is achieved by temporarily suppressing the
incompatibility reaction using one of the following techniques.
Bud Pollination. Bud pollination means application of mature pollen
to immature nonreceptive stigma, generally 1-2 days prior to the
anthesis of dowers. This is the most practicable and successful
method both in the gametophytic and sporophytic systems. In some
cases, application of the fluid from mature stigmas may improve the
success of bud pollination.
Surgical Techniques . Removal of the stigmatic surface, the whole of
stigma or a part or whole of the style may permit an otherwise
incompatible mating. Removal of the stigma is very useful in the
sporophytic system, e.g. % Brassica , while removal of the style is help-
ful in some cases of gametophytic incompatibility, e.g.. Petunia . In
Petunia, the whole of the style may be removed and the pollen grains
may be directly dropped on to the ovules in the ovarian cavity.
60
Plant Breeding : Principles and Methods
End- of- Season Pollination. In some species, the degree of incompati-
bility is reduced towards the end of the dowering season or in mature
plaints. But there are controversial reports on the usefulness of this
technique.
High Temperature . In some species, e.g., Trifolium, Lycopersicm ,
Brassica , Oenothera etc., exposure of pistils to temperatures upto
66°C induces pesudo-fertility.
Irradiation . In the single-locus gametop&ytic system, e.g., in
Solatiaceae, acute irradiation with X-pays or gamma-rays induces a
temporary loss of self-incompatibility.
Grafting . Grafting of a branch onto another .branch of the same
plant or of another plant is reported to reduce the degree of self-
incompatibility in Trifolium pratense . There is only one report
on this phenomenon, and the machanism of this reduction is not
known.
Double Pollination . In some species, self-incompatible matings
become possible when incompatible pollen is applied as a mixture
with a compatible pollen, or it is applied after pollination with a
compatible pollen.
Other Techniques . A number of other techniques have been tried with
varying degrees of success, ’ but they are not commonly used. These
techniques are : treatment of flowers with carbon monooxide,
injecting styles with immunosuppressants, application of electrical
potential difference of about 100 V between the stigma and pollen
grains, treatment of pistil with phytohormones and with protein
synthesis inhibitors, and steel brush pollination.
MALE STERILITY
Male sterility is characterised by nonfunctional pollen grains ,
while famale gametes function normally. It occurs in nature
sporadically, perhaps due to mutation. Male sterility is classified into
three groups : (1) genetic, (2) cytoplasmic, and (3) cytoplasmic-
genetic.
Genetic Male Sterility
Genetic male sterility is ordinarily governed by a single reces-
sive gene, ms, but dominant genes governing male sterility are
also known, e.g., in safflower. Male sterility alleles ' arise
spontaneously or may be artificially induced. A male sterile line
may be maintained by crossing it with heterozygous male fertile
plants. Such a mating produces 1 : 1 male sterile and male fertile
plants (Fig. 3.6)*.
Utilization in Plant Breeding - Genetic male sterility may be used in
hybrid seed production. T fie progeny from, ms ms X Ms ms crosses
are used as ferbale, and are interplanted with a hombzvgous male
Ratio 3 : 1
Modes of Reproduction and Pollination Control
' fertile (Ms Ms) pollinator. The genotypes of the ms ms and Ms ms
lines are identical except for the ms locus, i.e., they are isogenic, and
are known as male sterile (A) and maintainer (B) lines, respectively.
The female line would, therefore, contain both male sterile and male
fertile plants ; the latter must be identified and removed before
pollen shedding. This is done by identifying the male fertile plants in
seedling stage either due to the pleiotropic e fleet of the ms gene or
due to the phenotypic effect of a closely-linked gene. Pollen dispersal
from the male (pollinator) line should be good for a satisfactory
seed set in the female line. However, generally pollen dispersal is
poor and good, closely-linked markers are rare. Roguing of male
fertile plants from the female line is costly as a result of which the
cost of hybrid seed is higher. Due to these difficulties, genetic
male sterility has been exploited commercially only in a few countries.
In USA, it is being successfully used in castor. In India, it is being
used for hybrid seed production of arhar (C. cajan) by some private
seed companies, t\g., Maharashtra Hybrid Seed Co. Ltd., India,
produced and sold 50 Q seed of a hybrid variety of arhar. Suggestions
have been made for its use in several other crops, c.g.< cotton,
barley, tomato, sunflower, cucurbits etc., but it is not yet practically
feasible.
Parents
F,
F 2
Inheritance of Male Sterility
ms ms >: Ms Ms
(Male sterile) (Male fertile)
T
Ms ms
(Male fertile)
i
1 Ms Ms, 2 Ms mSi 1 ms ms
Male fertile
Male sterile
Maintenance of A Male Sterile Line
Male sterile .strain A
Cross the male serhe line
with the male fertile sib
line
o ms ms X Ms Ms S
Male sterile (line A) Male fertile
i (line B)
1 Ms ms : 1 ms ms $
Male fertile X Male sterile
(Seed not harvested) (Seed harvested)
i
$ 1 Ms ms : 1 ms ms $
Male fertile x Male sterile
(Seed not harvested) (Seed harvested)
Maintained indefinitely
Inheritance of genetic male strarility. Male sterility is produced by a
recessive nuclear gene designated as ms ; the dominant allele Ms pro-
duces male fertility. During the maintenance of male steviie lines,
sibmating is achieved through natural pollination.
The Fi (Ms ms) is back-
crossed with the ma b
sterile line A
Maintained by sib mating.
Seed from the male sterile
plants only is harvested.
Maintained by sib mating
Seed from the male sterile
p’ants only is harvested.
61 Plant Breeding ; Principles and Methods
Cytoplasmic Male Sterility
This type of male sterility is determined by the cytoplasm (Fig.
3.7). Since the cytoylasm of a zygote comes primarily from egg cell,
the progeny of such male sterile plants would always be male sterile.
Cytoplasmic male sterility is known in many plant species, some of
which are crop plants (Table 3.2). Cytoplasmic male sterility may
be transferred easily to a given strain by using that strain as a polli-
nator (recurrent parent) in the successive generations of a backcross
programme (Fig. 3.8). After 6-7 backcrosses, the nuclear genotype of
the male sterile line would be almost identical to that of the recurrent
pollinator strain. The male sterile line is maintained by crossing it
CYTOPLASM STERILE
NUCLEAR GENE NONRESTORER
IT IS UNABLE TO COUNTERACT
the effect of the Sterile
CYTOPLASM
CYTOPLASfrf FERTILE
nuclear gene non restorer
MALE FERTILE
MALE STERILE MALE FERTILE MALE STERILE
Fig 3.7. Cytoplasmic male sterility. Male sterility is caused by the cytoplasm
(depicted by S). The F cytoplasm is the normal cytoplasm. If a
restorer gene (the dominant allele of r) becomes available, the male
sterility will be included in the cytoplasmic-genetic group. This type
of male sterility shows strict cytoplasmic inheritance.
with the pollinator strain used as the recurrent parent in the back-
cross programme since its nuclear genotype is identical with that of
the male sterile line. Such a male fertile line is known as the main-
tainer line ox B line it is used to maintain the male sterile line.
The male sterile line is also known as the A line . There is considera-
ble evidence that the gene or genes conditioning cytoplasmic male
sterility, particularly in maize, reside in mitochondria, and may be
located in a plasmid like element.
Modes of Reproduction and Pollination Control
Transfor of Male Sterility to a New Strain
STRASfSJ A STRAIN 8
Cross the male
fertile strain B to
a cytoplasmic male
sterile strain A
(i) F, would be
male sterile
(ii) Backcross the
Fj to strain B
in) 50% o fthe
nuclear genes
would be from
strain B
male sterile
STRAIN b
(i) Backcross to
strain B
(ii) 75% of the
nuclear genes
would he from
strain B.
male sterile
strain b
Plant Breeding ; Principles and- Methods
Table 3.2. Cytoplasmic and cytoplasmic-genetic male sterility in some
important crop plants.
Crop species (nucleus) Cytoplasm Restorer genes
Remarks
Maize, (Zea mays) cms-C (Z. mays)* One (i?/ 3 )* *
Spontaneous ' re-
version relatively
high
cms-S (Z. mays)* One (Rf 4 )* *
cms-T (Z. mays)* Two (/?/i , Rfi)* *
Commercially
’used
Most’ commonly
used, slightly re-
tards growth and
yield (2-4%),
susceptibility to
Helminthosporium
leaf blight
Nicotiana tahacum Nicotiana debneyl
Nicotiana megalosiphon
Nicotiana bigelcvii
'Poor growth,
floral abnormali-
ties, commercially
not used.
Tritiatm aestivum Triticum timopheevii Two (Rfi , Rfd
Aegibps caudaui from T.. titno-
phee vii
Commercially
not used.
Triticum durum
Aegibps ovata
Commercially
not used
Gossypium hirsutum Gossypium anomahtm
Gossypium arbor cum
Commercially
not used
Sorghum bicolor
Milo ( S . bicolor)
One ( Msc )
Commercially
used
Pennisetum americanum Tifton (P. americanum )*
Ludhiana (P. americanum)*
Commercially
used
Helianthus annus
H. annuus*
Two (Rfi , Rfz)
Commercially
used ■
Wild iice (WA type) Eporophytic
O. sativa F. spontanea action
Chinsurah Boro II Gametophytic
(BT type) action
Gambiaca (Gam type)
O-Shan-Tao-Bai
Widely used
used
used
used
used
Brassica juncea
B, napus
Two genes,
cumulative
effect
proposed to
be used
Mutant cytoplasm,
Restoration of fertility by Rfz and Rf 4 shows sporophytic control, that is,
fertility of pollen grains depends on the genotype of the plant, while that
by Rf shows gametophytic control, he., fertility of pollen grains depends
Modes of Reproduction and Pollination Control 65
Utilization In Plant Breeding. Cytoplasmic male sterility may be utiliz-
ed for producing hybrid seed in certain ornamental species, or in
species where a'vegetative part is of economic value. But in those ■
crop plants where seed is the economic part, it is of no use because
the hybrid progeny would be male sterile.
Cytoplasmic-Genetic Male Sterility
This is a case of cytoplasmic male sterility where a nuclear
gene for restoring fertility in the male sterile line is known. The
fertility restorer gene, R s is dominant and is found in certain strains
of the species, or may be ■ transferred from a i elated species, e.g., in
wheat (Table 3.2). This gene restores male fertility in the male sterile
line, hence it is known as restorer gene (Fig. 3.9). The cases of
cytoplasmic male sterility would be included in the cytoplasmic-
genetic system as and when restorer genes for them would be dis-
covered. It is likely that a restorer gene would be found for all the
cases of cytoplasmic male sterility if a thorough search were made.
This system is known in maize, jowar ? bajra, sunflower, rice and
wheat (Table 3.2).
The plants would be male sterile in the presence of male
sterile cytoplasm if the nuclear genotype were rr, but would be male
fertile if the nucleus were Rr or RR (Fig. 3.9). New male sterile lines
may be developed following the same procedure as in the case of
cytoplasmic system (Fig. 3.8). But the nuclear genotype of the polli-
nator strain used in the transfer must be rr, otherwise the fertility
would be restored. The development of new restorer strains is some
what indirect (Fig. 3. 10). First a restorer strain (say R) is crossed with
a male sterile line (A). The resulting male fertile plants are used as
the female parent in repeated backcrosses with the strain (C), used as
the recurrent parent, to which the transfer of restorer gene is desired.
In each generation, male sterile plants are discarded, and the male
fertile plants are used as females for backcrossing to the strain
€. This acts as a selection device for the restorer gene R during
the backcross programme. At the end of the backcross programme, •
a restorer line isogenic to the strain C would be recovered.
Utilization in Plant Breeding. The cytoplasmic-genetic male sterility
is used commercially to produce hybird seed in maize, bajra and
jowar. A generalised scheme for producing a double cross in maize
is presented in Fig. 3.11. Alternatively, two fertility restoring inbreds
may be used to produce the single cross II. One of the two inbreds,
In this case, will have to be detasselled for use as the female parent*
Thus all plants in the double cross will be male fertile, since the
single cross II will be homozygous for the restorer gene R . For
producing single crosses, the scheme for production of the single
cross II as presented in Fig. 3.1 1 is followed. A triple cross may be
produced by crossing single cross 1 (Fig; 3.11) with a fertility restor-
ing inbred so that ail the plants in the triple cross would be male
fertile.
Plant Breeding : Principles and Methods
Cytoplasm sterile
Nuclear gene nonrestorer, /.<?., recessive allele of the
resorter gene
Cytoplasm fertile (nonsterile)
Nuclear gene nonrestorer
Cytoplam sterile
Nuclear gene restorer in homozygous (RR) or
heterozygous (Rr) state
Effect of the sterile cytoplasm negated by the
restorer gene
Various Genotypes and Phenotypes
male sterile
MALE STEELE
MALE FERTILE
MALE FERULE
.MALE STERILE
MAIL P£R T 'L C
) MALE FERTILE
t MALE STER'LE
Fig. 3.9.
Results of Various Matings
Cytoplasmic-genetic male sterility. Male sterility is due to a sterile
cytoplasm ; fertility is restored by a restorer gene, which is usually
a dominant nuclear gene. F/S in the cytoplasm indicates that the
cytoplasm may be either fertile (F) or sterile (S).
MALE STERILE
MALE FERTILE
1
BOTH MALE
FERTILE
Modes of Reproduction and Pollination Control
67
rl o'
seventh
V CAR
EIGHTH
Of TENTH :
YEAR
, male ferule ■
■ ; SELPrPOi. L -NATE O ..
PROGENY TEAT ; NONS6C-REGAT.no PROGENIES
S E IB C 75 £;S E ORE G A T i NG PR G C fr N if; O'SCAROEO .
Restorer line crossed to a
cytoplasmic male sterile
line. This allows selection
for R gene in the segregat-
ing generations.
The resulting male fertile
Fi is crossed to the strain
C to which R is to be
transferred The F* is used
as female to retain the
male sterile cytoplasm.
Male fertile, progeny is
back-crossed to strain C.
Strain C is used as the
recurrent male parent.
Male fertile progeny is
back-crossed to strain C.
Strain C is used as male.
Male fertile progeny self-
pollinated
Male fertile progeny selfed.
Individual plant progenies
grown in the next genera-
tion and nonsegregating
progenies selected.
Fig. 3 30. Transfer of restorer gene R from a restorer strain (strain R) to a
new strain (strain C).
Origin .of Male Sterile Cytoplasm. Male sterile cytoplasm arises
spontaneously in nature or may be produced by the breeder. The
various sources of the male Sterile cytoplasm are given below.
68
Plant Breeding : Principles and Methods
DOu&LB CROSS
|Ax8)x(CxD>
H 1 SLGttEOA T IOM
J OB MALL l UUlLI f Y/
STERILITY,
Fig. 3*1 1. A generalised sene me fur the production of a double cross in maize
using cytoplasmic-genetic male sterility.
JU Spontaneous Mutation . Mutant male sterile cytoplasms arise
spontaneously in low frequencies. Mutant cytoplasms have been
isolated in maize, bajra and sunflower (Table 2.2). It is likely that
an extensive search would lead to the isolation of male sterile cyto-
plasms in almost every crop species.
2. Interspecific Hybridization. Transferor the full somatic chromo-
some complement of a crop species, through repeated backcrossing,
into the cytoplasm of a related. wild species often leads to cytoplasmic
male sterility* Male sterile cytopiarns of wheat have been derived
from Triticum timopheevii or Aegilops caudata. Other examples are
listed in Table 2.2. in cross-pollinated crop species, the male sterile
cytoplasms have generally originated through mutation, while in
self-pollinated crops they have been transferred from related species.
3. Induction through Ethidium Bromide. . Ethidium bromide is a
potent mutagen for cytoplasmic genes or plasmagenes. Male sterile
cytoplasm may be induced by seed treatment with ethidium bromide.
Such mutants have been induced in some 'plant species, e.g.. Petunia.
Limitations of Cytoplasmic-Genetic Male Sterility for Use In Plant
Breeding. There are several problems in the utilization of cytoplasmic-
genetic male sterility in hybrid seed production in many species.
These problems are listed below.
1. Undesirable Effects of the Cytoplasm . Male sterile cytoplasms
generally have undersirable side effects (Table 2.2). For example,
the Texas cytoplasm (cms-T) in maize, by far the most successful
cytoplasm commercially, slightly retards growth, yield (2.4%),
Modes of Reproduction and Pollination Control 69
plant Height and leaf number ; induces earlier silking and delayed
pollen shedding ; and makes the plants highly susceptible to
Helminthosporium leaf blight. This susceptibility is due to the
extreme sensitivity of mitochondria from cms«T genotypes to a
toxin produced by the fungus- A good cytoplasm without any
side effects is indeed rare. The male-sterile cytoplasms in tobacco
could not be used due to their severe uodersirable side effects.
Restorer genes only restore male fertility ; they are unable to
remove the side effects of the male sterile cytoplasms,
2. Unsatisfactory Fertility Restoration . In many cases, restoration
of fertility is not satisfactory. As a result, these sources cannot
be used in the production of hybrid seed.
3. Unsatisfactory Pollination . Natural pollination is often not
satisfactory, except in wind-pollinated crops like maize. This
reduces the production of hybrid seed, and thereby increases its
cost. Id, some species, e.g.. Capsicum , this has prevented the use
of male sterility in hybrid reed production. Poor pollination
would always be a major problem in self-pollinators, e.g,, wheat
In rice, this problem is overcome by regular tripping of ears
in the morning, generally using a rope.
4. Modifier genes may reduce the effectiveness of cytoplasmic
male sterility. During backcrosses, while transferring the male
Sterile cytoplasms, the nuclear genetic 'background may also be
disturbed. This may lead to some pollen production by the male
sterile lines.
5. Sometimes, cytoplasm may also be 'contributed by the sperm
which, in the long run. may lead to a breakdown of the male
sterility mechanism.
6. Male sterility mechanisms may break down partially under
certain environmental conditions resulting in some pollen pro-
duction by the male sterile lines. This problem has been
encountered in maize, bajra and jowar.
7. In crops like wheat, polyploid nature of the crop and undesira-
ble linkages with the restorer gene make it very difficult to develop
a suitable restorer (R) line.
SUMMARY
-Crop plants reproduce asexually or sexually. Some crops show both
sexual and asexual reproduction. Sexually reproducing crops are either self-
pollinated or cross-pollinated. Self-pollination is ensured by cleistogamy,
• opening of flowers after pollination, clustering of anthers over stigma etc.
i ' Cross-poliination is promoted by raonoecy, dioecy. dichogamy, self-incompati*
bility or a combination of these mechanisms. Asexually reproducing and cross*
pollinated crops are highly heterozygous and generally exhibit strong inbreed-
ing depression., , The self-poll mated species are homozygous and do not show
inbreeding depression. Ihe different-breeding methods are primarily based on
the response of crop species to inbreeding. Thus in self-pollinated species
a variety consists of homozygous plants, while in cross-pollinated species
heterozygosity has to be retained or restored. Thus the mode of reproduction
70 Plant Breeding : Principles and Methods
of crop plants determines their genetic constitution, their response to
inbreeding, ease in pollination control and the stability of varieties after
release.
Self-incompatibility may be heteromorphic or homomorphic. Hetero-
morphic system shows sporopbytic control of the pollen incompatibility reac-
tion. But the homomorphic system exhibits both gametophytic and sporo-
phytic control. Male sterility is either genetic* cytoplasmic or cytoplasmic-
genetic. Both incompatibility and male sterility are useful in the production of
hybrid seed. The cytoplasmic-genetic system has been the most commonly
used in hybrid seed production.
QUESTIONS
1. What do you understand from mode of reproduction ? . Discuss its rele-
vance to plant breeding programmes with the help of suitable examples.
2. Briefly describe the various modes of reproduction prevalent in crop
plants.
3. What are the differences between the following : (i) Parthenogenesis and
apomixis, (ii) Gametogenesis and sporogenesis, (iii) Self-incompatibility
and male sterility, and (iv) Often cross-pollinated and cross- pollinated
species.
4 . Define sporogenesis. Briefly describe the process of micro- and mega-
sporogcnesis wi th the help of suitable diagrams.
5. Describe in brief the process of gametogenesis in plants.
6. Write short notes on the following : (i) Apomixis, (ii) Parihengenesis,
(iii) Self-incompatibility, (iv) Male sterility, (v) Triple fusion, (vi) Betero-
styly, (vii) Anthesis, (viii) Often cross-pollinated species, (ix) Cleistogamy,
(x) Restorer gene, (xi) Chasmogamy, and (xii) Dichogamy.
?. Describe the various mechanisms which promote cross-pollination (or
ensure self-pollination) in plants.
8. How would you determine the mode of reproduction of a species ? Briefly
describe the procedure for determining the amount of cross-pollination in
a species.
9. Give a suitable classification for self- incompatibility. Describe the diffe-
rent types of self-incompatibility with the help of suitable examples.
10. What is male sterility ? List the various types of male sterility found in
plants. Explain the cytoplasmic-genetic male sterility with the help of
suitable examples and diagrams.
11. Discuss the relevance of self-incompatibility and male sterility in plant
breeding. Briefly explain the limitations of each system.
12. In the field, a single plant of a crop species does not set seeds/pods. How
would you decide whether the lack of seed set in this plant is due to male
sterility, self-incompatibility or some other reason ?
13. How would you maintain the following ? (i) A self-incompatible line,
(ii) A genetic male sterile line, (iii) A cytoplasmic male sterile line, (iv) A
cytoplasmic-genetic male sterile line, and (v) A restorer line.
14. Define self-, cross- and often cross-pollination. Discuss their effects on
the genetic composition of populations and their relevance to crop impro-
. ' vement \ ,
Suggested Further Reading
Allard, R.W. 1960. Principles of Plant Breeding. John Wiley and Sons, Inc.,
New York.
Arasu, N.T. 1968. Self-incompatibility in Angiosperms ; a review, Genetica
39:1-24.
Modes of Reproduction and Pollination Control **
Duwick, D.W. 1965. Cytoplasmic pollen sterility in core. A dv. Genet.
13 ; 1-56.
Edwardson, J.R, 1970. Cytoplasmic male sterility. Bot, Rev. 36 : 341-420.
Elliot, F.C 1958. Plant Breeding and Cytogenetics. McGraw-Hill Book Co.,
Inc.* New York.
Frankel, R. and Galun, E. 1977. Pollination Mechanisms* Reproduction and
Plant Breeding. Springer, Berlin, Heidelberg, New York.
Gustafsson, A. 1968. Reproduction mode and crop improvement. Theor.
Appl. Genet. 38: 109-117.
Harvey, P.H., Levings, C.H. and Wernsman, E.A. 1972. The role of extra-
chromosomal inheritance in plant breeding. Adv. Agron. 24 : i-28.
Heslop-Harrsion, I. and Lewis, D. (eds.). 1975. A Discussion on Incompati-
bility In Flowering Plants. Free. Royal Soc. London B, 188 : 233-375.
Lewis, D. 1942. The evolution of sex in flowering plants. Biol. Rev. 17 : 46-67*
Foehlman, J.M. and Borthakur, D.N. 1969, Breeding Asian Field Crops with
Special Reference to Crops of India. Oxford and 1BH Publishing Co.,
New Delhi.
CHAPTER 4
Qualitative and Quantitative
Characters
The two basic requirements of plant breeding are variation and
selection. Variation in a character is a must for an improvement in
that character. If all the plants in a population were identical for a
character, no improvement in that character would be possible. In
such a case variation must be created through hybridization, muta-
tion or polyploidy (see later). Selection is the second basic step in
crop improvement. Selection involves the identification and isolation
of desirable plants . The progeny from selected plants may be released
as a variety if they are found suitable and superior to the existing
varieties.
The selection of plants from a population is almost always based
on theix appearance, i,e„ phenotype . Phenotype has both heritable
and nonheritable components. The heritable component is due to the
genes present in plants, that is, genotype . The nonheritable com-
ponent consists of the effects of environment. The value of progeny
obtained from a selected plant, therefore, 'would largely depend upon
the relative contributions by the heritable and nonheritable compo-
nents to its phenotype. Clearly, the breeder should be thoroughly
familar with the laws of inheritance and the relative importance of
the genotype and the environment in determining the concerned
phenotype.
In the foregoing chapters, and indeed throughout this text, it
has been assumed that the reader is familiar with the elementary
laws of genetics. However, the laws of genetics that concern us the
most are briefly reviewed here. Phenotype is the appearance of a
plant with respect to a particular character, such as plant height,
flower colour etc., or it may refer to a group of characters. The
development of a character is mainly governed by one or a group of
genes. But the genes do not produce the characters' directly. The
genes, produce different proteins, which often act as enzymes. These
enzymes catalyse specific biochemical reactions. - Thus the primary
function of a gene is to produce a specific enzyme (more precisely, a
polypeptide) which catalyses a specific biochemical reaction*
a?#
73
Qualitative and Quantitative Characters
These biochemical reactions finally lead to the development of
■ various characters of the plant, that is, the phenotype. The
relationship between the primary (enzyme production) and the
ultimate (phenotype production) functions of a gene is well
understood for a limited number of characters, while for
most of the characters it is not known. The genes and the characters
they produce are often separated by several biochemical and physical
events. Thus character development is likely to be modified by
the action ofLother genes in the genotype of a plant, and also by the
environment. However, the degree to which the gene function is
affected by these factors varies considerably from one character to
the other.
Some characters are little affected ^by other genes, le. p the
genetic background, or the environment. Such characters are gene-
rally governed by one or few genes with large, easily detectable
effects. These genes are known as oligogenes. The characters pro-
duced by oligogenes show distinct classes and are known as qualita-
tive characters. On the other hand, the development of many charac-
ters is very much affected by the genetic background and, more
particularly, by' the environment. These characters are governed by
several genes with small individual effects ; these genes are known as
polygenes. The characters produced by polygenes are referred to as
quantitative characters , because they do not show clear-cut classes
and have to be studied by measurement. Inheritance of both quali-
tative and quantitative characters follows the laws of Mendel. But
the effects of individual genes in the two cases are totally different.
Consequently, the techniques used to study the two types of charac-
ters are also different. These differences stem primarily from the
ease in classifying plants into distinct phenotypic classes in one
case and the inability to do so in the other. In crop improvement,
both the types of characters are important. Many characters of
economic value show the qualitative, while several others exhibit the
quantitative mode of inheritance.
INHERITANCE OF QUALITATIVE CHARACTERS
Mendel proposed the laws of inheritance based on his studies
with qualitative characters. * Subsequently, a large amount of addi-
tional information has been collected to supplement the laws of
Mendel. It is universally accepted that the characters are determined
by genes, and that genes are normally located in chromosomes
which are present in the nucleus of cell. In body ceils or somatic
cells of higher plants, each chromosome has an identical partner or
homologue . During meiosis, the two homologous chromosomes pair
and move to the opposite poles at the first anaphase. The gametes
produced, therefore, receive only one of the two homologue s of each
chromosome.' Fertilization restores the somatic chromosome number
in zygotes since a .zygote is produced by fusion of two gametes, one
male and the other female.
Apparently, in sombre cells each gene has two copies present
in the two homoiogues. The two copies may be identical or they
74 Plant Breeding : Principles and Methods
may be different. The behaviour of genes during gametogenesis and
fertilization is identical with that of the chromosomes. Each charac-
ter has two, sometimes more, alternative forms known as contrasting
characters. For example, seed shape in peas ( Pisum sativum) m ay be
round or wrinkled. The contrasting characters are determined by
alternative forms of the same gene which are referred to as alleles.
When two alleles are brought together through an appropriate cross,
generally one of the alleles expresses itself, while the other is unable
to do so. The allele that expresses itself in the presence of another
allele is dominant , and the one that is unable to express itself is
recessive. For example, the allele for round seed shape (W) may be
brought together, with that for wrinkled seed shape (w) by crossing
a plant producing round seeds with one having wrinkled seeds. The
seeds resulting from this cross (Fi) are round. Thus allele W expresses
itself at the expense of the allele w. Therefore, W is dominant and w
is recessive.
At the time of gamete formation , the two alleles present in the
Fi (Ww) separate and pass into different gametes ; this is known as
segregation. Segregation of the two alleles of one gene produces two
types of gametes, e.g., W and w, in equal frequencies. Random
union among these gametes produces three different types of zygotes
wrinkled
ROUND
Ww
ROUND
Fig. 4.1. Inheritance of round and wrinkled seeds in pea.
Table 4.1. Frequency of different genotypes and phenotypes produced by
segregation of a single gene Ww (round Vs. wrinkled seeds) in peas.
Phenotype
round
round
wrinkled
Genotype
Frequency
Frequency
i
/
w
\
W
w
ww
Ww
ROUND
round
W
Ww
ww
ROUND
WRINKLED
Qualitative and Quantitative Characters 75
(WW, Ww and ww) which produce two phenotypes (round and
wrinkled) in the ratio 3 : 1 (Fig. 4.1, Table 4.3). When two or more
genes are segregating together, often their segregation is independent
of each other, ? e., at the time of gamete formation an allele of one
gene goes with each of the two alleles of the other gene or genes with
equal frequency. In case of two genes, independent segregation
produces four different types of gametes in equal frequencies.
Random union among these gametes produces nine genotypes which
give rise to foqr phenotypes in the ratio 9 ; 3 : 3 : 1 (Fig. 4.2,
Table 4.2).
parents WW GG X ww 99
round yellow wrinkled green
Ww Gg
ROUND YELLOW
WwG'G WwGg
ROUND ROUND
YELLOW YELLOW
WW GG WW Gg
round round
YELLOW YELLOW
WW Gg wwgg WwGg ' Wwgg
round round round round
YELLOW GREEN YELLOW GREEN
WwGG WwGg wwQG wwGg
ROUND ROUND WRINKLED WRINKLED
YELLOW YELLOW YElLOW YELLOW
WwGg
round
YELLOW
Ww gg ww Gg
ROUND WRINKLED
GREEN YELLOW
wwgg
WRINKLED
GREEN
Fig. 4.2. Independent segregation of seed shape (round/wrinkled) and
seed colour (yellow/green) in pea.
When the two copies of a gene (alleles) present in an individual
are identical, e.g., WW or ww, it is known as a homozygote . But when
two alleles are dissimilar, e.g., Ww, it is termed as a heterozygote .
Both homozygous and heterozygous conditions produce the same
phenotype, e.g,, round seed, when there is full dominance. Thus the
same phenotype may be produced by different genotypes. The homo-
zygous (WW) and heterozygous (Ww) round-seeded plants cannot
be separated on the basis of their phenotypes. But they can be easily
differentiated by studying their progency, ie., from their breeding
76
Plant Breeding : Principles and Methods
Table 4.2* Genotypic and phenotypic frequencies produced by independent
segregation of two nonaiSetes J'f'wand Gg (round vs. wrinkled and
yellow vs. green seeds) in pea.
Genotype
Frequency
Phenotype
Frequency
WWGG
1
round yellow
9
WWGg
2
round yellow
WwGG
2
round yellow
WwGg
4
1 round yellow
WWgg
I
round green
3
Wwgg
2 .
round green
wwGG
i
wrinkled yellow
3
wwGg
2
wrinkled yellow
vwgg
1
wrinkled green
1
behaviour. The determination of genotype or genotypic value of a
plant by studying the progeny produced by it is known as progeny
test. Progeny test is an essential part of all modern plant breeding
programmes.
PLEIOTROPY
Generally., each oligogene is supposed to govern a single char-
acter. But there are several instances where a single major gene (oligo-
gene) is known to affect more than one character. The phenomenon
of a single major gene affecting more than one character is known as
pieiotropy , and such a gene action is known as pleiotropic gene action .
For example, a case of pieiotropy has been reported in wheat for a
gene producing awned spikes. It was found that addition of the
awned character to an awnless variety Onas, by recurrent backcross-
ing, increased the yield as well as seed size. Removal of the awns
from the awned variety Baart had a comparable but the opposite
effect.
Clearcut cases of pieiotropy are but limited. This is mainly
due to the difficulty in demonstrating the pleiotropic effects of a gene.
To investigate pieiotropy, two lines must be developed which are
identical with respect to all other genes except for the gene under
investigation. Such lines are known as isogenic lines. Isogenic
lines may be developed in two ways. In the first method , Fj and the
successive generations from a suitable cross are selfed. In each
generation, heterozygotes for the gene in question are- selected.
After 8-10 generations of selling, such plants may be expected to be
homozygous for all the genes except for the gene in question. The
two homozygotes isolated from the progeny of a single heterozygous
plant at this stage may be expected to differ only for the gene under
study. The second method of producing isogenic lines consists of
recurrent back-crossing for 10 or more generations. Thus in a study
of pieiotropy, about 10 generations are required to produce isogenic
lines, obviously a time consuming process. Isogenic lines have "been
extensively used to investigate pleiotropic effects of many genes in a
number of crop species.
77
Qualitative and Quantitative Characters
Pleiotropy may result from true plelotropic gene action or it
may be due to a closely-linked gene or a group of genes. The linkage
may often be sufficiently close to keep the genes together during the
production of isogenic lines. Therefore, even the study of isogenic
lines does not establish pleiotropy beyond doubt. Ao example of this
situation is furnished by the rough and smooth awns in barley
(Hordeum vulgar e ). Semismooth-awned derivatives from the rough-
awned Atlas barley out-yielded Atlas as well as the rough-awned
derivatives. The "semismooth and rough- awned derivatives were
obtained by 10 backcrosses to Atlas. But a careful study later
revealed that the higher yielding ability of - the semismooth-awned
derivatives was due to linked genes and not due to pleiotropy.
in crop improvement , it makes Utile difference whether the
favourable side effects of a gene are due to pleiotropy or due to tight
linkage . But when the side effects are unfavourable , true pleiotropy
offers no chance of removing these side effects. But in the case of tight
linkage , the side effects may be removed by raising sufficiently large
populations to recover the recombinants lacking these side effects .
PENETRANCE AND EXPRESSIVITY
Generally, oligogenes express themselves in all the individuals
that carry them, and their expression is fairly uniform. But some
oligogenes fail to express themselves in some of the individuals
carrying them. Such genes are said to have incomplete penetrance .
Thus penetrance is the ability of a gene to express itself in the indivi-
duals carrying it in the appropriate genotype . For a dominant gene,
the appropriate genotype will be the dominant homozygote as well
as the heterozygote, while for a recessive gene it will be the recessive
homozygote. For example, there is a gene that causes partial chloro-
phyll deficiency in the cotyledonary leaves of Lima beans (Phaseolus
lunatus ). But only about 10 per cent of the seedlings carrying this
gene show chlorophyll- deficiency. This gene* therefore, has a pene-.
trance, of 10 per cent, i\e., it expresses itself in' 10 per cent of the
individuals carrying it. On the other hand, the gene which expresses
itself in every individual, that carries it is said to have a complete
penetrance . , ■ ,, ’
g Some oligogenes, in contrast, show variable degrees of expres-
sion in different individuals having the appropriate genotype for this
gene. Such genes are said to have variable expressivity . Expres-
sivity, therefore , denotes the ability * of a gene to express itself uni-
formly in all the individuals that carry it * A gene that expresses
itself uniformly in all the individuals has uniform expressivity , while
those that are unable to do so have variable expressivity . The gene
producing chlorophyll deficiency in the Lima bean (P. lunatus) seed-
lings exhibits variable expressivity in addition to incomplete pen-
etrance. In some seedlings, the cotyledonary leaves have no chloro-
Plant Breeding : Principles and Methods
phyll, in the others the chlorophyll is absent from the leaf tip, while
in still others only the margins of leaves are affected. Thus a single
gene with variable expressivity may produce a number of phenotypes
as if more than one gene were involved. Almost always, genes
showing incomplete penetrance also show variable expressivity. In
fact, incomplete penetrance is the consequence of variable expres-
sivity when the phenotype produced in some of the individuals is so
small as to be undetectable, incomplete penetrance and variable
expressivity are the consequences of the effects of environmental
factors on the expression of concerned genes.
Incomplete penetrance and variable expressivity confuse the
relationship between genotype and phenotype . Consequently , such
genes pose difficulty in selection of desirable types . In such cases ,
progeny tests for more than one generation may be required to estab-
lish the genotype of the plants beyond a reasonable doubt .
THRESHOLD CHARACTERS
Some genes require a specific environment, e.g. f a particular
temperature, for their expression. Characters whose development
depends upon a specific environment are known as threshold charac-
ters. For example, a mutant gene in barley ( Hordeum vulgare) pro-
duces albino seedlings at temperatures below 8°C. But when the
seedlings carrying this gene are grown at I9°C or above, they deve-
lop into normal green seedlings. Incomplete penetrance of some
genes may be due to a threshold requirement.
The knowledge of threshold may be helpful in the identification
of desirable genotypes. For example, disease resistant and disease
susceptible plants cannot be differentiated unless the plants are
exposed to the concerned disease. Similarly, lodging resistant and
lodging susceptible plants can be separated only when conditions
that favour lodging are present. Many other similar instances
may be encountered in crop improvement.
MODIFYING GENES
There are many genes that do not seem to have any effect of
their own, but they increase or decrease the expression of other
oligogenes. These genes are known as modifying genes since they
modify the effects of other genes. Individual modifying genes have
small effects which are cumulative in nature. Consequently, their
effects are quantitative in nature. Modifying genes are generally
studied in the same manner as quantitative characters. It is
likely that most of the major genes are affected by a group of
modifying genes. For example, spotting in mice is affected by modi-
fying genes and has been thoroughly studied. A dominant gene
S is necessary for spotted colour. But the effect of S is modified by
several modifying genes that affect the degree of spotting. The
modifying genes produce almost a continuous series from uniform-
black to uniform white. Thus the action of a major gene may be con-
siderably affected by the genetic background in which it is present.
* \
Qualitative and Quantitative Characters
19
Similarly, the degree of self-incompatibility produced by different
S alleles is considerably affected by the genetic background
(modifying genes). Many disease resistance genes are also known to
^ be affected by the genetic background.
In crop improvement , while transferring a major gene , the
breeder should be careful to retain , eliminate or accumulate modifying
genes according to the needs of the situation. It has been suggested
that the crop improvement may soon reach a stage when breeders
will have accumulated all the available major genes. Further
improvement would then largely depend on the accumulation of
favourable modifying genes.
GENE INTERACTION
%
Many, perhaps most, qualitative characters are determined b'
two or more oligogenes. These genes show various relationships
with each other in producing the character. These relationships are
known as gene interaction . In Japanese varieties or rice (O sativa),
two dominant genes, Rc and Rd f are needed to produce red pericarp.
When one of the two genes or both the genes are in the recessive
state, the pericarp is white. This is known as comlementary gene
action , and shows a 9 : 7 ratio in F 2 . In maize (Zea mays), a gene
Pr has no effect of its own. But Pr modifies the action of R 9 which
produces red colour. In the presence of i?, Pr changes the red colour
to purple. This type of gene interaction is referred to as supplemen-
tary gene action ; it produces a 9 : 3 : 4 ratio in F*. Inhibitory gene
action is characterised by an inhibitory gene which stops the action
of another gene, and has no effect of its own. Red colour produc-
tion in maize is stopped by an inhibitory gene I ; the recessive allele
i is hot inhibitory. The Fa ratio in this case is 13 ; 3.
Another gene interaction, often known as epistatic gene action ,
produces 12 : 3 : 1 ratio in F 2 . In barley ( Hordeum vulgare ), a
dominant gene Y produces yellow seed coat, while another gene B
produces black seed coat. When genes B and Y are together, the
effect of Y cannot be seen due to the masking effect of B, i e., black
colour. It is more appropriate to denote this gene action as masking
gene action since epistasis is generally used to denote interaction
between two nonallellic genes, irrespective of the manner in which
they interact.
Some characters are governed by two genes that produce
identical effects, whether together or alone. The alternative form
of the chraracter results only when both the genes are in the recessive .
form. The ratio in F ; 2 is 15 : 1, and the interaction is' known as
duplicate gene action. For example, the nonfloating habit in rice is
governed by two genes, Dw x and Dw& either alone or together.
The floating habit results only when both the genes are in the
recessive form, i.e., dw x dw t dw 2 </w 2 - Some genes show polymeric
80 Plant Breeding : Principles and Methods
gene action. In this case, the two genes produce identical effects
individually, but their effect is increased when both the genes are
present together. In barley, gene A or B alone produces awns of
medium length. But when A and B are present together, long awns
result. The double recessive condition, aa bb, produces awnless
lemma. The ratio in F s is 9 : 6 : 1.
Often more than two genes may by involved in an interaction,
and these genes may have different relationships with each other.
This leads to much more complex Fs ratios. However, if one under-
stands the various interactions between two genes as described above,
one could work out the various ratios that may be expected in such
complex interactions.
The chemical basis of gene interaction has been worked out in
several cases. Gene interactions arise because each character is
generally produced by a series of biochemical reactions. In this
chain of biochemical reactions, each reaction provides the substrate
for the next reaction. Each reaction is usually controlled by one
gene. Each gene generally produces an enzyme which catalyses a
biochemical reaction . A simple gene interaction situation maybe
as follows. If a gene controlling a reaction is in recessive state, it
would not produce an active enzyme ; hence the reaction controlled
by this gene would not take place. The reactions occurring beyond
this one in the reaction chain would also not occur due to a lack of
substrates even though the genes governing them were present in
the respective dominant form. This interaction would produce com-
plementary gene action. For example, HCN (hydrocyanic acid)
production in clovers (Trifolium sp.) is governed by two dominant
genes, say, A and B. Gene A controls a reaction that produces a
cyanogenic glycoside. The enzyme produced by the gene B releases
HCN from this glycoside. Thus HCN production would require the
presence of both the genes A and B in the dominant state.
If the gene A were in recessive state, the cyanogenic glycoside
would not be produced, and there would be no substrate
for the enzyme produced by gene B. And if the gene B were in
recessive state, HCN would not be released from the cyanogenic
glycoside produced by gene A.
LINKAGE
So, far, we have assumed that the genes segregate indepen-
dently. However, genes often show a tendency to be inherited together,
that is, a tendency to pass to the same gamete during segregation,
and do not show independent segregation. This phenomenon is known
as linkage. The genes that show linkage are situated in the same
chromosome. Each chromosome is transmitted intact as a unit during
meiosis. Consequently, the genes situated in a chromosome are
also transmitted together. But during meiosis, there is exchange
of chromatin material between homologous chromosomes ; this
is known as crossing over. Crossing over, therefore, leads to
Qualitative and Quantitative Characters SI
recombination between linked genes. The frequency of recombi-
nation between any two- linked genes depends upon the distance
between them. Thus the chief effect of linkage is to reduce the fre -
auency oj recombination between linked genes (Fig. 4 3, Table 4.3).
COUPLE
10 PHASE
REPULSION PHASE
<
<
“0
X vp / vp
Vp/ Vp X
vP/ vP
TWO-ROW
SIX- ROW
TWO-ROW
Six- ROW
PURPLE
WHITE
WHITE
PURPLE
©
©
(Vp)
©
TWO-ROW
PURPLE
< vp/ vp
SIX- ROW
WHITS
Test* cross progeny
Phenotype
VP/ vP X V P/ V P
TWO-ROW
PURPLE
SIX-ROW
WHITE
Frequency ( %)
Coupling Repulsion
VP VPjvp Two*row, purple 40.3 9.7
vp vpJvp Six-row, white 40.3 9.7
Vp Vp'.vp Two-row white . 9.7 403
vP vPl vp Six-row, purple 9.7 40.3
*In barley, six-row condition of spikes is recessive to two-row condition,
and white lemma colour is recessive to purple lemma colour.
Fig. 4.3. Inheritance of number of rows in the spike and lemma colour in
barley.
Linkage may be in- coupling phase In which two dominant
genes are linked together, e.g., VPjvp , or in repulsion phase where
one dominant and one recessive gene are linked, e.g. 9 Vp! vP t The
coupling or repulsion phases alter drastically the frequencies of
various phenotypes, in F 2 and other segregating generations but
have no effect on the frequency of recombination (Fig. 4.3, Table
4.3).
Linkage is of considerable value In plant breeding. When two
favourable genes are linked , they tend to be transmitted together , and
ore easily combined in the progeny. In case of a tight linkage crossing
over less than one per cent), selection for only one of the cham-
bers would be necessary. This may be an ad vantage in cases where
selection for one of the characters is easier than that for the other.
For example, r esistance for stem rust and loose smut are closely
lin ked in b&rlef fH. yufzareY. The techniques of inocufation For stem
fu st are much easier than those for loose "smut. TterBfoieT'tn a
breeding programme, the breeder may select for stem rust resistance
only. The loose smut resistance would be selected for due to its
close linkage with stem rust resistance. But a linkage between a
desirable and an undesirable gene causes delay in crop improvement
programmes . The breeder would have to grow a sufficiently large F%
WiB,
m:
8 1 Fkmt Breeding : Principles and Methods
Table 43. Effect of linkage in coupling, and repulsion phases on the recovery
of AB'AB genotype from Fa.
Cross-over
value (p)*
Frequency of A BIAS individuals in if
(in percent)
' Coupling (A Blab) .Repulsion (a if Ah)
0*50 (independent segregation)
6.7 5
6.25
0*40
9.00
4.00
0.30
12.25
2.25
0.25
14.06
1.56
• 0 20
16,00
1 00
0.10
20.25
0,25'
0.05
22,56
0 0625
0.02
24.01
0.01
0.0 1
24.50 ,
0*0025
P
1 00 X f;j (1 J?) 2 J
1 00 x C-iF 2 )
* p represents the proportion of cross-over gametes produced by the K
(double heterozygote A Blab or AbjaB ). This proportion is con-
veniently determined by studying testcross (Fixab'ab) progeny.
The proportion of recombinant types in the testcross progeny is/?.
In coupling phase, the recombinant types would be Ab and aB,
while in the repulsion phase they would be AB and ab.
population to allow the recovery of desirable recombinants . The size
of required Ft would depend upon the closeness of linkage ; the closer
the linkage , the larger the size of F z . Certain wheat (T. aestivum)
strains, such as, Cl 12633, have a gene for stem rust resistance from
Triticum timopheevii. This gene is closely Jinked with late maturity.
This linkage was broken by growing a largp population to recover
early rust resistant recombinants. ! f. f -
QUANTITATIVE CHARACTERS
Quantitative characters are governed by^ several genes ;
each gene has .small effect which is usually ciim.ulat.ive* These charac-
ters are considerably affected by the environment. Quantitative
characters show a continuous variation and it is not possible to
classify them into distinct classes,. The examples of such characters
are yield, plant height, days to flower, days to maturity, protein
content, seed size, etc. If a large number of .observations on a
quantitative character are plotted in a frequency distribution curve,
often a bell-shaped normal curve would be obtained. Since classfi-
cation of individuals into distinct classes is not possible, inheritance
of quantitative characters cannot be studied through classical tech-
niques of genetic analysis applied so successfully to the study of
qualitative characters* Therefore, inheritance studies on quantitative
characters have to employ statistical 'procedures. A special branch
of mathematics, Biometry , has developed as a result of the construc-
tion of special models and procedures to deal with the various
aspects and problems of quantitative inheritance.
Qualitative ami Quantitative Characters 83
THE MULTIPLE FACTOR HYPOTHESIS
The inheritance of .quantitative characters was a matter, of
great controversy for a considerable time, Francis Gallon and his
followers considered that the Mendel’s laws of inheritance could not
explain the continuous variation of such characters. Others, notably
Hugo de Vries, regarded continuous variation to be nonherltable.
! r , 1906, Yule suggested that many genes with small, similar. effects
could produce continuous variation. He proposed that these genes
were transmitted according to the laws of Mendel.
In 1908, NilssorbEhle presented experimental evidence to
support the hypothesis of Yule. ■ He studied the inheritance of seed
BRENTS
GAMETES
RiRi R2R2
DARK RED
nr, r 2 r 2
WHITE
/
Hifi R 2 r 2
medium red
r.r 2
R,r a
ft R2
r ir 2
R1R1 R2R2
R1R1 R 2 r 2 '
R 1 r 1 R 2 R2
Rlf-i R.2^2
DARK RED
MEDIUM
DARK RED
MEDIUM
DARK RED
MEDIUM
RED
■ . Ri Ri R 2 r 2
R 1 R t r 2
Rifi R 2 r 2
Rifi r 2^ 2
medium
OARK RED
medium
RED
medium
RED
LIGHT
RED
^j,n 2 R 2
MEDIUM
• DARK RED
Rth $ 2^2
MEDIUM
RED
r 1 r 1 R2R2
MEDIUM
RED
1 r t R 2 r 2
LIGHT .
RED
RiTi R 2 r2
Rifi r 2 r 2
. ' f if% R.jf 2..:. ,
i*i fi fjfs
MEDIUM
RED
LIGHT
BED
LIGHT
RED
WHITE
Fig. 4.4. Inheritance of seed colour in
1
84
Plant Breeding : Principles and Methods
Table 4.4 Genotype and phenotype frequencies produced by the segregation
of two genes with cumulative effect, R 1 r l R 2 i% for seed colour in
wheat.
Genotype
Frequency
Nttmbe* of
dominant
alleles
Phenotype
Frequency
RiRi RzRz
1
4
Dark red
1
Rin RsRt '
2
3
Medium-
dark red
4
RiRi R‘ir 2
2
3
Medium-
darkred
Rin Rzn
4
2
Medium red
6
RiRi r 2 r*
1
2
Medium red
rin RzRz
I
2
Medium red
Rin
2
1
Light red
4
nn R*r *>
2
1
Light red
nri nr%
!
0
White
1
colour in wheat (Triticum sp.) and oats (A vena sp .). The F 2 genera-
tions from various crosses had red and white grains in the ratios of
3 : I, 15 : 1 or 63 : !. It is clear that the seed colour in these crosses
was governed by one, two and three genes, respectively. For further
analysis, let us consider the cross giving 15 red : 1 white seeds. A
closer study of seed colour revealed that the seeds classified as red
differed in the intensity of colour. The red seeds could be grouped
into four distinct classes : dark red, medium-dark red, medium red
and light red. These four classes were in the ratio 1 : 4 : 6 : 4. Thus
the 15 :• 1 ratio was actually 1 : 4 : 6 : 4 : 1. It can be seen from Fig.
4.4 and Table 4.4 that this ratio is obtained if it is assumed that the
seed colour is governed by two genes with simitar , small and additive
effects . The dominant alleles of the two genes produce red colour,
while the recessive alleles produce no colour. The intensity of
colour depends on the number of dominant alleles present. Thus
genotypes RiRi r 2 r 2) r x n R Z R, 2 and Rm i? 2 r 2 will produce the same
intensity of colour since they all have two .dominant alleles. Thus
Nflsson-EMe was able to show that certain characters are governed
by genes that have small and cumulative effect . This, in essence, is
the multiple factor hypothesis or pologenic inheritance .
The data of Nilsson-Ehle demonstrated that genes possessed
the necessary qualities, Le.> small and cumulative effect, to explain
continuous variation. But the evidence was not conclusive because
the character he studied exhibited discontinuous variation. East in
1916 presented conclusive evidence that quantitative characters are
governed by many genes with small and cumulative effect. He
studied the inheritance of corolla length in Nicotiana longiflora . The
two parents had contrasting corolla lengths, viz., 40 and 93 inm,
and the F\ was intermediate with 63 mm long corolla. The F 2
showed a continuous variation for corolla length which could not
PfiffeENT OF PLANTS
Qualitative and Quantitative Characters
'3 {INDIVIDUAL PLANT PROGENIES)
'4 (INDIVIDUAL PLANT PROGENIES)
Plant Breeding- i Principles and Methods
be grouped into separate classes* To analyse bis data, East used
statistical parameters, such as, frequency distribution, mean and
coefficient of variation. The; essential features of hh study are
summarised below and presented In Fig. 4.5.
1. The two parents were inbreds and almost homozygous. But
there was variation within the two parents for corolla length- This
variation, therefore, may be assumed to arise from environmental
effects.
2. The Ft mean was intermediate between those of the two parents.
This would be expected if multiple factors with cumulative, effect
governed this trait.
3. The variability in F 5 was comparable to that; In the parents.
Again, all the F l plants would have the same genotype. There-
fore, the variability present in F 3 , was obviously produced by the
environment.
4. The F s generation was much more variable than the parents and
the F h East postulated that the greater variability in F% resulted
from the segregation and recombination of Mendelian genes.
Thus it Is clear that these observations correspond closely to
the expectations based on Mendelian inheritance. It Is not possible
to determine the number of genes controlling the character. But as
. few as 5 genes with cumulative gene action would produce a conti-
nuous curve in Fg. Although with 5 genes there wpuld be only 1 1
classes, each class separated by 4*5 rum, the identity of these classes
would disappear due to the effects of environment as is evident from
the data on the parents and F 2 .
Confirmatory evidence for Mendelian inheritance of corolla
length may be obtained from Fe 9 F 4 and F s generations. Based on
Mendelian principles, ip may be predicted that,
1, F 2 plants with markedly different corolla lengths should produce
F 3 progenies that differ from each other.
2. Different F z progenies would show different • variabilities. ' Some
F 3 progenies should be as variable as the F%> while others would
be less variable. Variability in some Fs progenies may be com-
parable to that in the parents. These expectations arise due to the
differences in the genotypes of different F 2 plants ; some are
comparable to the Fi, .while some are more like the parents
(Fig. 4.4). . '
3, In F s and the succeeding generations, variability of a progeny
would be equal to or less than that of the progeny from which it
arose, but never more. This is because a plant present in a F s
progeny «
produced
gosify '
4. Since F 3 i
; expected
'V. Igeoeratio.
Qualitative md Quantitative Characters
87
should also decrease with the advancing generation, Le.,
n if, xit of variability Fd> ;c.
The data, from f s# jF 4 and F s generations agreed closely with
the above foist predictions. The-'JFi families differed significantly for
mean corolla length. The mean corolla length of F z families was
associated with the corolla length of the jF a plants from which they
were derived. The variability pattern in F Zs F& and F\ : also followed
th predicted pattern These observations tend to confirm that the
* oroita W* gfh *s f *o erned by several genes with small and cumulative
effect. It may be pointed out that polygenes (multiple factors) do
aot always have additive gene action! They-' ; generally show dermi-
w ,ce and ?pistatic actions. as well,
„ C'L mmC INHERIT Ah C£ AND CONTINUOUS
VARI/il liijN "
Tfe chief difference between qiudietive and qualitative traits lies
in the degree to which they are affected by the environment. Qualitative
characters are little or not at all affected by the environment, while
quantitative characters are considerably affected. The chief effect of
environment is toynask the differences between different genotypes
and to produce a continuous variation, in the character,, It can be
shown that a character governed by a single gene with additive effect
would show discrete distribution 'when the genotype alone determines
the phenotype, ?>., there is no effect of the environment on the phe-
notype. As the magnitude of the effect of environment on the pheno-
type increases, the phenotypic classes progressively overlap each
other and form a continuous curve. The distribution becomes roughly
similar to the normal curve when the contribution of environment is
50 per cent If the contribution of environment increases to 75 per
cent, the distribution tends to approach normal distribution. ■Experi-
mental* evidence suggests that m case of most of the quantitative
characters, the contribution of environment varies from 10-50 per
cent, while in some cases, such as yield, it may often be much higher.
Thus environmental ^effect should'. produce a continuous variation even
if the number of genes governing a character was very small 9 or even
• A similar continuous variation will be produced if the number
of wmss governing the character is increased f and it is assumed that
the effect of environment is zero As the number of genes is in-
creased, the number of phenotypic classes, also Increases! If one
assumes 12 pairs of genes, there would be 25 phenotypic classes, and
i w distribution would lend to become continuous With cumulative
gene effect* t re distribution would be symmetrical, *\e., similar to the
norma! distribution. Even with complete dor nuance, the distribution
would be dose to the normal one. La practice, the effect of environ-
ment would never be zero. Therefore, the distribution will be a
t ' o 's om* c en h cc rnplete dominance.
The' multiple factor hypothesis or polygenic inheritance would
produce continuous distribution, more. specifically /normal distrxbu-
Plant Breeding $ Principles and Methods
tion, provided' the number of genes governing the character is large
and the character is affected by environment. This conclusion
would be correct even in the case of complete dominance. The poly-
genic inheritance of quantitative characters is universally accepted.
But it is now recognised that the alleles of polygenes may show
dominance in addition to the additive effect, and nonalleles may
show interallelic interactions of different types A series of mathe-
matical models has been developed to estimate the importance of
allelic and nonallelic interactions in quantitative inheritance.
ROLE OF ENVIRONMENT IN QUANTITATIVE
INHERITANCE
Quantitative characters, by definition, are considerably affected
by environment. The main result of this effect is that the relationship
between genotype and phenotype is partially or completely hidden,
i,e, } the phenotype does not reveal the genotype. For example, if
the environmental effect is zero, the phenotype would be produced
by the genotype, only. Thus there will be complete correspondence
between phenotype and genotype. However; the effect of environ-
ment is seldom zero. As a result, the phenotype is the result of a
joint action of genotype and environment. Obviously, the effect of
environment is not heritable and it cannot be passed on to the
progeny. Thus only that part of phenotype that is the result of •
genotype is heritable.
In crop improvement, the breeder selects plants on the basis of
their phenotype. The effectiveness of selection would largely depend
on the proportion of phenotype due to the genotype. Therefore, it
is important for the breeder to know the extent to which environ-
ment influences different quantitative characters. The estimate of the
effect of environment on a character may be obtained from a simple
experiment. The experiment consists of a large number of genotypes,
he., strains, varieties etc., grow a in a replicated trial. The number of
genotypes should be as large as possible, but it should not be less
than 30. The data thus obtained are subjected to analysis of variance
appropriate for the experimental design used. The analysis of
variance based on a Randomised Block Design is given in Table 4 5.
Table 4.5, Analysis of variance for genotypes grown in a replicated trial
according to a Randomised Block Design.
Source of variation d.f Expectation of MS
' {degrees of freedom) (mean squares)
Genotypes 6"—l v % e+R<s*g ■
Replications R — 1 o 2 e+Gd*r
Error (ft-l) (0-1 ) eV
Total . •' (RGh-l ■ ' ' ■
Where, G and R are the numbers of genotypes and replications, respectively ;
and cr 2 <?„ oV and a*g denote the variances due to composite error* replication
and genotype, respectively.
Qualitative and Quantitative Characters
The phenotype, may be described accordiag to a mathematical
jnodel to facilitate statistical analysis and interpretation. The pheno-
typic mean, Le. $ x a given, genotype from the trial may be ex-
pressed as,
X^P+g+e+ge
where, F is the general population mean, that is the mean phenotype
of all possible genotypes grown under all possible environments ; g
is the effect of genotype ; e is the effect of environment ; and ge de-
notes the interaction between genotype and environment. The geno-
type X environment interaction signifies that the relative performance
of various genotypes is affected by the environment. In other words,
the relative performance of the genotypes would change if the
environment is changed. For example, genotype A may be superior
to genotype B in one environment, but in another environment it
would be inferior to B. However, if genotype X environment interac-
tion is absent, genotype A will be superior (or inferior) to B in all
the environments.
Using the mean squares (variances) from an analysis of
variance table (Table 4.5), an estimate of the contribution of geno-
type to the phenotype may be obtained. The mean square due to
genotypes has an expectation of v 2 e+M 2 g, The & 2 e represents the
remainder mean square or a composite error term, which includes
differences la plot effects within a replication, sampling error, and
errors in measurement. The ® 2 g> on the other hand, represents the'
component of variance due to genetic differences among the geno-
types. Thus the genetic component of variance may be estimated
as follows.
Genotypic variance (® 2 g)~(MS due to genotypes - MS dm to
error)/i?.
. Where, R is the number of replications*
=[($%+ — a*e]/R
^Ra*g/R
Similarly, phenotypic variance (<r*p) can be estimated by totalling
the genotypic and environmental variances.
Table 4.6. Analysis of variance for nitrogen content per plant in mung F.{
radiatd) grown on agar slants In symbiosis with M05 cowpea
; Rhizobium. Since the agar slants do not contain combined nitro-
gen, the nitrogen content of the plant Is a reliable measure of its
nitrogen fixing ability in symbiosis.
Source of variation
d. j
f.
Sum of squares
Mean
( SS )
squares {MS)
Genotypes
48
519.419
10.821
Replications.
2
■ 0.082
0.041
Error
96
12.294
0.128
90
Plant Breeding ■; Principles and Methods
Since, in practice, the MS due to genotype* and error arc based on a
sample, it is more appropriate to use the symbols F„ and V,. m place
of a 2 u and ffV The method may be illustrated by using the analysis
of variance data presented in Table 4.6. Table 4.6 is based on data
on total nitrogen content per plant in mung genotypes planted in a
Randomised Block Design with three replictations.
V t ~{MS due to genotypes— -ATS due to error)//?
==(10.821 — 0-T 28>/3
= 10.693/3 or 3.564
Similarly,
Vv«= F e + Vi (Ve is the MS due to error)
-3.564 + 0. 128
**3.692
An alternative method of estimating the genotypic variance
consists of growing the two parents (Pi and P £ ), the F x and the F :t in
a trial. Variances are estimated for P u P% Fi and F-z separately,
la the case of Pi, P„ and F u the variances are due to environment
only since all the plants within each of them have the same geno-
type. The variance for Ft, on the other hand, has both genetic and
environmental components. The environmental variance ( V?) may
fee estimated as follows.
V,**(V Pl +Vp t +VFi)/3
and the variance for F 2 may be written as.
¥p^ V,+ V»
Therefore, V„~ Vez—{(Vn + Vp s + Ffj)/3]
This is illustrated by using data on days to .flowering m case of bajra
( Pennisetum aniericanum).
Fft=23.Q2 ; Fpj= 10.19 ; F/> a =9.29 ; and TV, =9.46
Therefore, F,= 23.02 -[( 10.1 9 + 9.29 + 9.46)/3J
=23.02-9.65 ■ • T ■ ' *
^ =13.37
It is apparent that the effect of a genotype can be estimated
only in relation to other genotypes, i.e., in relation to u. Therefore,
the performance of genotypes should be evaluated under a number
of environments to obtain reliable estimates cf genotypic and
pfaenotyic variances. The most accurate estimates would be obtain-
ed if all possible genotypes were tested under all possible environ-
ments. But in practice, this is simply not possible. The experimenter
usually attempts to investigate the effects of relatively more impor-
tant environmental factors, such as, temperature, rainfall, soil fertility
etc. Generally, these factors are considered to be represented by
locations and years, and the breeder does not study the effects of
individua 1 environmental factors. This is done because of the near
impossibility of manipulating and controlling the various environ-
mental factors over the experimental area. If desired, the breeder
may evaluate the effects of easily manageable factors, such as,
irrigation water, fertilizer application, population density, date of
Qualitative and Quantitative Characters 91
planting, etc. However, generally the experiment is conducted at
two or more locations in two or more years. In case of such an
experiment, the mean phenotype is given by,
~X —p+g+r+l+y-trgl-^-gy+ly+gly-i-e
where r, l and y denote the effects of replication, location and years,
respectively, and e is the composite error term. Interaction between
genotype and locations is represented by gl, between genotype and
years by gy, between locations and years by ly, and among
genotypes and locations and years by gly.
Therefore, when genotypic variance is estimated from a trial
conducted at only one location in one year, it also contains the
interaction variances, i,e., it is the sum of trig, c 2 gl, a 2 gy and a-gly.
Therefore, the genotypic variance estimated from a single trial is
biased upward. The interaction variances (<r*gl, a 2 gy, a 2 gly) can be
estimated from trials coducted at several locations during several
seasons. The genotypic variances estimated from such trials would
be unbiased and raliable.
COMPONENTS OF GENETIC VARIANCE
The genetic variance estimated from trials using homozygous
lines arises from differences between homozygotes. But the breeder
is mostly concerned with segregating populations, e.g., Ft, F„ etc.,
derived from crosses. In such cases, the genotypes used in the study
are not homozygous, but they will be homozygous for some genes
and hetrozygous for the others. In case of only additive gene
action, it does not matter whether a gene is homozygous or
heterozygous and with, what other genes it is present. But strictly
additive gene action is. an oversimplification of the real situation j
the polygenes show dominance as well as epistatic gene actions.
} Fisher in 1918 divided the genotypic variance into three com-
ponents : (1). additive, (2) dominance and (3) interaction components.
AdditiVe Component. It is the component arising from differences
between the two homozygotes for a gene, e.g,, AA and aa.
Dominance Component. Dominance component is due to the
deviation of the' hetrozygote (Aa) from the averages of the two
homozygotes (A A and aa). This is sometimes referred to V as intra-
alletic interaction.
Interaction or Epistatic Component. The epistatic component results
from interaction between two or more genes.
Later, Hayman and Mather partitioned the epistatic component
into three types of interactions, viz., additive X additive, additive X
dominance and dominance ,< dominance. The various components
of gene action, their notations and descriptions are summarised in
Table 4.7. .
The primary objective of biometrical genetics, a branch of
genetics concerned with the application of biometry to gbnetic
■ ■
1
Plant Breeding : Principles and Methods
is to identify and estimate the various components of genetic
Several techniques are available for this purpose, but each
t suffers from one limitation or the other (see Chapter 5).
ie 4.7, Different types of gene actions involving two genes Aa, Bb.
ESTIMATION OF COMPONENTS OF GENETIC VARIANCE
The additive and dominance components of genotypic variance
may be estimated from a simple experiment. The experiment con-
sists of two oarents. Pi and P%, their F\ and i- 1, and tne two back-
crosses, Bi (Fj X Pi) arid B^(F ixP 2 ). The variances from these
generations are used to obtain estimates of additive and dominance
variances. These estimates are based on the assumption that epis-
tasis is absent. The environmental component of the phenotypic
variance is estimated by averaging the variances of Pi, Ps and Fi,
the nonsegregating generations.
The theoretical basis for estimating the additive and dominance
components of variance may be explained by taking an example of
a single gene A with two alieles A and a. Three genotypes are
possible for this gene which may be represented, as follows (Fig 4.6),
MID PARENT
Fig. 4.6. Relationship between homozygotes and heterozygotes
at* a single locus (midparent is the average of AA and aa).
Component of genetic
variation
' Specific Symbol
Description General
Symbol
Additive
da
The difference between AA d
and aa
db
The difference between BE
and bb
Dominance
ha ■
The deviation of Aa from k
the average of AA and aa
. kb
The deviation of Bb from
the average of BB and bb
Epi stasis
da x db
Additivex additive effect i
due to interaction between
A A a nr* •
dax kb and
ft ft
Additive x dominance inter- j
ha x db
action, due to interaction
between AA and Bb, and
Aa and BB , respectively
ha X kb
Dominance x dominance .in- i
ternction due to interaction
between Aa and Bb
Qualitative and Quantitative Characters 93
The symbol da denotes the additive effect of the positive allele
A, while -da is the additive effect of the negative allele a . 'The posi-
tive and negative signs denote the direction of effects of the two
alleles on the phenotype. Since there will be more than one gene
affecting a character, the phenotype of a homozygous line would be,
*Y*=2(-j-<30+2I(— d)-\~c ,
where, 2+rf is the additive effect of positive alleles at all the loci
affecting the character, 2 - d is the additive effect of all the negative
alleles, and c denotes the effects' of the genotypic background and the
environment. Thus the phenotype of a pureline would be the sum
total of positive and negative additive effects of all the genes affect-
ing the character, and the value c. For a single locus, if A= 0 (there
Is so dominance), the Ft will be intermediate between the two
homozygotes. But quantitative characters are governed by more
than one gene. Therefore, if F t dees not deviate from the mid-
parent value (that is, average phenotype of the two parents) it does
not necessarily mean that dominance is absent. It may simply mean
that dominance effects of different genes have opposite signs and
thus cancel each other out, i.e . 9 2/z=0. When all the positive alleles
are present in one strain and all the negative alleles are present in the
other, the two parents would differ by 2 2 d, because one parent will
have the phenotype 2d+c a*nd the other will have the phenotype
2-rf+c. . But in practice, both strains would contain some positive
and some negative alleles. Consequently, the difference between
them would be always less than 2 2d.
On the basis of above considerations, we may estimate she
variances of different segregating genetations with respect to a single
gene, A a, as follows :
Generation, Fs : Genotype , AA Aa aa
Phenotype , da ha — da
Frequency , \ \ |
Thus, Mean = \da + \ha ~~\da (since the population size is 1
= | ha
Sum of squares—! {da)*+\ (ha) 2 +K — dd) 2
And sum of squares of deviations
from F% mean^ [liddF+lihay+l^— da)*] — {\hdf
= Q da 2 F \ha x ) ~iha 2
==%da*+iha 2
Since we are dealing with a population of size l f the sum of squares
is also the variance of F %> ' Since there will be more than one gene
affecting the character, the formula may be rewritten as follows.
^(Variance of F 2 )—
For simplicity, we may replace 'Sd* with Z>, and 2h z with H. Since
there will be an environmental component, E 9 we may add E to the
equation to complete the formula for Yf%«
V^iP+iH+E
Plant Breeding s Principles and Methods
Similarly,, for tine backcrosses.
Generation, B\ : Genotype
Phenotype
Frequency . ..
Thus, 'mem *» 1 da + 1 ha
Sum of squares—
and, sum of squares of deviations
from Bi » » *
*s 4 J&j* 4-" J Ili0® — ~ 4 d 5 hd.
The above is also V® since we size of population is 1. Summation
m ' *» «t. - rat IS t «* »/*»
-<fcl
over ail the toci and addition of the environmental effect wm gi ve,
Fat- i&P+iSA* - Ptf/i+E
Similarly, for the other backcross.
Generation, B% : Genotype : i<3
Phenotype : ha
Frequency : \ §
Thus, mean — § /ta + 1 ( — da)
= iha— \da
Sum of squares=i(^a) 2 +i(- ^)“
and, sum of squares of deviations
from B, mea'tt-ti(Afl)*+ K - da)') - Q ha -Way
«= [Iha'+lda^—ilha'+lda* - \da ha)
!</a ha
This is also V B% , as in the case of V Bl . Summation over ail the loci
and addition of the environmental component would give,
¥g„— I2k tj r}2d*+ \1dh + E
'=IH+ID+Hdh+E
When we add the variances for Bi and B 2 generations, we get,
V Bt + V Bi =(iD+iH-iIdh+E)HlD+iH+l'2sik-tE)
=$D'\~sH-{’2E
Thus the variances of the different segregating generations may
be summarised as follows.
Vf^ID + IH+E
VBi + Vs 2 =iD+iH+2E
From these equations, we can estimate D as follows.
2VF,~D+m+2E
-(V*±V*,)-i D+ IH+2E
2V Ft - Vb x +Vb?=°\D
An estimate of the environmental variances can be obtained by
averaging the variances of the nonsegregating generations.
Qualitative and Quantitative Characters
Estimation of B and H is iilust:
flower isi bajra if. amerkmmum ),
Therefore,
The estimate of E is gives by, :
E=(V Pl + V Ft + V Pl )/3
=(10.I9-f 9.29-f-9.48)/3
= 9.65
Finally,
=23.02-9.06-9.65
=4.31
Therefore, #=17.24
HERITABILITY
la crop improvement, only the genetic component of variation
is important since only this component is transmitted to the next
generation. The ratio of genetic variance to the total variance, i.e.,
phenotypic variance, is known as heritability. Thus heritability deno-
tes the proportion of phenotypic variance that is due to genotype,
t.e., heritable. Heritability may be represented as follows.
Heritability (H)=VJV P
or = F„/(F„+F,)
where, V„, F„ and Fe are the genetic, phenotypic and environmen-
tal components of variances, respectively. •
Heritability can be estimated by three different methods listed
below.
1. From analysis of variance table of a trial consisting of a large
nupsber of genotypes.
96 Plant Breeding : Principles and Methods
2. Estimation of V a and Ve from the variances of F 2> P v P s and F l
, Parent-offspring regression upon doubling provides an estimate
ofheritabiiity. Thus H=2b, where b is the regression of progeny
means on parental values* . .
The use of analysis of variance data for estimating heritably
is «&?&<* !*«• ^ S o h ?2 8 ” %
the estimate of Vg in this case is ^.564 and that ot =- a u
heritability, therefore, will be,.
H=V a !(V„+V t )
=3.564/(3.564+0.128) or 3.564/3.692
=» 0.914
When expressed in per cent, H—91. 4 percent. _
The second method of estimation ofhentabihty consists oy he
estimation of Vo and Ve from variances of F s , Pi, P% ana *1 *s
explained previously. The heritability is then estimated using the
formula H=V'jl(V s -pVe).
Heritability estimated from the above three methods is
known as broad sense heritability. Broad sense hentaoriity esti-
mates' a«-e valid when homozygous lines are studied. However, when
we are 'dealing with segregating generations, the genetic variance
consists of additive and dominance components (assuming there is
no epistasis). Since in seif-pollinated crops we develop homozygous
lines the dominance component will not contribute to the phenotype
of the homozygous lines derived from the population. Consequently,
5n such cases only the additive component (D) of the genetic
variance is relevant. Therefore, the ratio of additive component of
variance to the total phenotypic variance is a more appropriate
estimate of heritability, and it is referred to as narrow sense
heritability . ....
For the estimation of narrow sense heritability, the components
of genetic variance are estimated by the procedure described earlier
in this chapter, or by some other suitable technique. The appropriate
value of D is used to determine heritability.
y F =|£>+iJT+£' (F Fa is the phenotypic variance of F a )
The value of D is estimated as described before. The heritability is
calculated as follows :
H(ns)=iDlVp 1
— 9.05/23.02
=0.394 or 39.4 per cent
Only i D is used for the estimation of heritability since the pheno-
typic variance of F-, has only iD as its component.
Estimates of heritability serve as a useful guide to the
breeder. The breeder is able to appreciate the proportion of variation
that is due to genotypic (broad sense H) or additive (narrow sense
H) effects, that is, the heritable portion of variation m the first case.
97
^ -w ;
and the portion of genetic variation that is fixable in porelines in the
latter case. If heritability of a character is very high, e.g 0.8 or
.more, selection for such a character should be fairly easy. This is
because there would be a close correspondence between genotype and.
phenotype due to a relatively smaller contribution of environment to-
the phenotype. But for a character with low heritability, say less
than 0.4, selection may. be considerably difficult or virtually im-
practical due,to the masking effect of environment on the genotypic
effects.
SUMMARY
The various characteristics of plants miy be qualitative or quantitative
an nature. Qualitative characters show distinct classes, are little affected by
the environment (both external and internal), and are governed by one or few
genes with large distinct effects, /.*?., oiigogenes . The quantitative traits, on the-
other hand, show continuous distribution, are generally influenced by the en-
vironment, and are controlled by several genes with small and cumulative'
effec t, i.e. 9 polygenes. Classical genetic analysis is based on study of qualita-
tive traits. Mendel showed that when two alleles are brought together In the
hybrid, they segregate at the time of gamete formation. Usually, nonalleles,
(two different genes) segregate independently, but often they may be linked, -
i>. s they may show a tendency to be inherited together. Linkage redacts the-
recovery of recombinant genotypes. Thus a linkage, between two desirable
genes is. an 'advantage in plant breeding. But linkage between one desirable
and one undesirable gene presents problems In crop improvement.
Oiigogenes generally govern a single trait, but sometimes they may affect
several unrelated traits ; this is known as pleiotropy. Ciearcut cases of pleiotropy
are limited ; many cases may be due to linked genes. Some genes are unable
to express themselves in all the individuals that carry them {incomplete pene-
trance, ), while others show variable expression {variable expressivity ). Certain
genes require a specific environment for their expression ; such characters are
known as threshold characters . Such genes present problems in selection. .
Action of many oiigogenes may be affected by modifying genes , which change
the intensity of expression. Modifying genes have no known action of their
own, except the modification of the action of the major gene. Many qualita*
tive traits are controlled by two or mpre oiigogenes which interact in various
wavs. The various interactions produce different ratios in Fa. eg, 9:7,
9 :‘3 : 4, 13 : 3, 12 : 3 : 1, 15 : 1 and 9:6:1. Often more than two genes may-
be involved leading to a complex relationship among the genes.
In 1903, Niisson-Ehle put forward the multiple factor hypothesis to ex- ,
plain the inheritance of seed colour in - wheat. According to this hypothesis,
some characters are governed by several genes with small and cumulative
effect. This would produce a graded variation which would tend to become
continuous due to environmental influences. East, in 1916, presented conclusive
evidence that quantitative characters are governed by multiple factors or
polygenes . This view is universally accepted, but it is recognised that the
. polygenes may show dominant and epistatic gene actions in addition to cumu-
lative gene effects.
The phenotype of quantitative traits is considerably InSuenced by en-
vironment ; it has environment and genotype x environment interaction com-
ponents. Variance due to environment may be estimated from a repficatcd
trial consisting of several genotypes. But the interaction component may be
estimated only when the trial is conducted in more than one environment, pre-
ferably at different locations and during different years. From such a trial*
the genetic and phenotypic variances can be computed, which may be used to
Plant Breeding : Principles and Methods
98
,, Va'iVe+Ve), denotes the proportion of
The genetic variance ^® 0 g U actlon^h' Thes^cornpoIentT of genetic
additive, dominance znd • ® n ‘appropriate biometrical analysis. An
variance can be estimated by u^ing an app^p domijnance effects can be
estimate of variances due to DUre lines (parents), their Fi, ts ami me
obtained from a trial co "? l * , ' n * ‘ °w s epistasis is assumed to be absent. From
zpssss
selection.
QUESTIONS
Differentiate iSSJWteW?
SSW^tSSRS'JSSSw^ -a
* s:?ssjs5r»-"
3 - ^»3wass: S“«““ d bsratt
shold characters and (vii) Epistasis.
4 Franci s G al iam
and Mather. # .
5 - KdiWtodWiC^hle h Shot that quanStatWe'charartiarewS
inherited according to the Mendelian laws .
generations ?
* S8£SX'»»2
fhe henSty for a trait is ICO per cent or zero per cent.
Suggested Farther Reading
Allahb, r.w. 1960. Principles of Plant Breeding. John Wiley and Sons, Inc.,
New York.
Allard R.W. and Bradshaw, A.D. 1964. Implications of genotype-envmon.
mental interactions in applied plant breeding. Crop Sci. 4 . 503-508.
Bailv, T.R. and Comstock, R E. 1476. Linkage and synthesis of better geno-
types in self-fertilizing species. Crop Sci. 16 . 363-370.
Becker. W.A. 1975. Manual of Quantitative Genetics. Student Book Crop.,
Pullman, Washington, U.S.A.
East, E.M. 1916. Studies on size inheritance in Nicotiana. Genetics 1 : 16447 .
Falconer, D.S. 1960. Introduction to Quantitative Genetics. Oliver and Boyd,
Edinburgh.
99
'Qualitative and Quantitative Characters
Fisher, R. A. 1918. The correlation between relatives oo the vSiappositiori of
Mendelian inheritance. Trans. Royal Soc. Edinburgh, 52 : 399.-433.
Gilbert, N.E. 1973, Biometrical Interpretation. Clarendon Press, Oxford.
Hayman, B I. and Mather, 1955, The description of genetic interactions in
continuous variation. Biometrics 1 1 : 69-82,
Hill, J. 1975. Genotype-environment interactions-- a challenge for plant breed-
ing. J. Agric. ScL, Cambridge, 85 : 477-494,
Mather, K. 1943. Polygenic inheritance and natural selection. Biol. Rev.,
18 : 32-64.
Mather, K. and Jinks, JX. 1977. Introduction to Biometrical Genetics,
Chapman and Hall, London,
.Moll, R.H. and S tuber, C.W. 1974. Quantitative genetics— empirical results
relevant to plant breeding. Adv, Agron. 26 : 277-314.
Biometry or biometrics is the science that deals with the applica-
tion of statistical procedures to the study of biological problems,,
Similarly, biometrical genetics is that branch of genetics which
attempts to unvavel the inheritance of quantitative traits using,
statistical concepts and procedures; it is also knows as quantitative-
genetics for obvious reasons. The various statistical procedures
employed in biometrical genetics are called biometrical techniques *
Several books aod a huge literature are available on the concepts-
aod the procedures of biometrical techniques. The present discussion
is limited in scope to a brief description of some common biome-
trical techniques and their relevance to plant breeding. It is intended
to outline the type of genetic- information obtainable from each
biometrical technique, and the manner in which that information is
helpful in plant breeding programmes.
Biometrical techniques are useful to the plant breeders in four
different ways : (1) in the assessment of genetic variability present in
a population (range, variance, standard deviation, coefficient of
variation, standard error, D 3 statistic, metroglyph analysis), (2) in
the selection of elite genotypes from mixed populations (correlation,
path and discriminant function analyses), (3) in the choice of parents
and breeding procedures (dialiel, partial diallel, line x tester, genera-
tion means, trialiel, quadriallel, biparehtal cross and triple test cross
analyses), and (4) in determining the varietal adaptation (stability
analysis).
ASSESSMENT OF VARIABILITY
An insight into the magnitude of variability present in a crop
species is of utmost importance as it provides the basis for effective-
selection. The total variation (phenotypic variation) present in a
population arises due to genotypic and environmental effects.
Phenotypic variability is the observable variation present in a
character in a population ; it includes both genotypic and environ-
mental components of variation and, as a result, its maguitude differs
under different environmental conditions. Genotypic variation, on
the other hand, is the component of variation which is due to the
genotypic differences among individuals within a population, and i&
the main concern of the plant breeder.
Biometrical Techniques in Plant Breeding lOt
Fisher was the first to divide, in 1918, the genetic variance into
three components, Le,, additive, dominance and episfatic variances.
Additive variance refers to that portion of genetic variance which is
produced by the deviations due to the average effects of the alleles
of the genes at all segregating loci. Dominance variance arises due
to the deviation from additive scheme of gene action resulting from
intraallelic interaction, that is, dominance. And episfatic variance
arises due to the deviations as a consequence of interallelic inter-
action.
The epistatic variance was further divided by Hayman and
Mather into the following three- components ; (1) additive X additive,
.(2) additive X dominance and (3) dominance X dominance interactions
(see, Chapter 4, p. 92). Wright suggested the partitioning of genetic
variance into two components, viz,, additive and nonadditive (domi-
nance and epistatic components), of which only the additive
■component contributes to genetic advance under selection.
Mather divided the^ phenotypic variance into three components,
namely, (!) heritable fixable (additive variance), (2) heritable non-
dSxable (dominance and epistatic components), and (3) nonheritable
nonfixable (environmental fraction). In fact, the heritable fixable
component of the phenotypic variance will include the additive X
additive fraction of the epistatic variance as well. Further, the total
phenotypic variance may be partitioned as (1) fixable (additive and
additivexadditive components) and (2) non-fixable (dominance,
epistatic minus additivexadditive and environmental fractions)
•components,
The above discussion may be summarised as follows :
cr*P=c 2 G+<* 2 E;
o*G=*0*A+am+v 2 I; and
ff*I= c 2 AA + c 2 AD+ff 2 DD
where, c 2 P«=» phenotypic variance, ff 2 G= genotypic variance, ff 2 A«*
additive variance, ff 2 D —dominance variance, cr 2 I— epistatic variance*
<j # A A« additive X additive variance, cr 2 AD=additive x dominance
variance, and ff 2 DD=dominance X dominance variance.
In a random mating population with no epistasis and zero
inbreeding, the covariance between a parent and its offspring is,
i* 2 A ; the covariance among half-sibs is, £a 2 A; and the covariance
•among fuil-sibs is Jo 2 A+io 2 D. These relationships change with
the level of inbreeding in the population.
Numerous studies have been conducted to estimate the magni-
tudes of various components of the genetic variance. The following,
general conclusions may be drawn from these studies. Genetic varia-
bility for important agronomic traits in almost all the crops is mainly
due to the additive genetic variance . The non-additive variance also’"
exists in nearly all crop species and for many important traits, but if
is generally smaller in magnitude than the additive component
The variability present in breeding populations can be assessed
m the following three ways : (1) using simple measures o
102
Plant Breeding : Principles mid Methods
L
2 .
(2) by estimating the various components of variance, and (3) by
studying the genetic diversity.
GSmnia & Measures of Variability. They include range, variance,
fSard^deviation (SD), standard error (SE) and coefficient of
variation (CV), which may be defined as follows.
RflIliie It i s the difference between the lowest and the highest
values present in the observations included in a sample.
Arithmetic Mean. It is also known simply as mean or average,
and is computed by dividing the sum of all the observations in
a sample by their number. Thus,
X=(2X)/ N
where x = mean, X=summation of, X=any observation in a
sample, and N=number of observation in the sample.
Variance It is expressed as the sum of squares of the devia-
rions of all observations of a sample from its mean, and
divided by the degrees of freedom (=N-1). It is generally
denoted by s 2 or v for the estimates from samples, and as <r-
for those from populations. It may be represented as follows.
j‘“=[2X 2 - {(SX) 2 /N}]/(N- 1)
Standard Deviation. It is the square root of variance, and is
designated as s or SD (in case of sample), or as a (in case of
population). Thus,
5 or SD = vV*
Coefficient of Variation. It is an important measure of varia-
bini y ‘ agSl Tcoinpiohly represented by CV. CV is the per cent
ratio 'of the standard deviation of a sample to its mean. Thus,
CV=(SD/x )X 100
From replicated data, phenotypic, genotypic and environmental
coefficients of variation can be estimated.
Standard Error. It is the measure of the mean difference bet-
ween sample estimate of mean (x) the population parameter
(M, i e.. it is the measure of uncontrolled variation present in a
sample. It is estimated by dividing the estimate of standard
deviation bv the square-root ol the number ol observations in
the sample, 'and is denoted by SE. Thus,
SE-SD/Vn
6 .
The magnitude of variation may also be assessed by estimating
the components of total variance of a sample. This involves crossing
a number of genotypes or strains in a definite fashion and the evalua-
tion of progeny thus obtained in replicated trials. Dial lei , partial
diallel, line X tester, generation means etc. analyses are used to
estimate the components of genetic variance. Thesejtecbniques will
be discussed separately later in this chapter.
The jhird' approach for assessing variability involves the study
of genetic diversity within a population. Two important biometrical
techniques, viz., D 2 statistic and metroglyph analysis, are commonly
103
Biometrical Techniques in Plant Breeding
used for this purpose. A brief description of these techniques is
given below.
Ijr Statistic. The variability among different genotypes of a species
p known as genetic diversity. Genetic -diversity arises either due to
Geographical separation or due to genetic barriers to crossability.
Variability differs from diversity In the sense that the former has
observable phenotypic differences, whereas the latter may or may not
have such an expression. One of the potent techniques of measuring
genetic divergence is the statistic proposed by Mahalanobis in
1936, if his technique measures the forces of differentiation at two
levels, namely, intraduster and intercluster levels, and thus helps in
the selection of genetically divergent parents for their exploitation
in hybridization programmes*
Genetic diversity plays an important role in plant breeding
’> because hybrids between lines of diverse origin generally display a
greater heterosis than those between closely related strains. For
example in fescue, the magnitude of heterosis increased with the
genetic divergence in morphological traits and flowering time,, and
also with the geographical origin of the parents. The magnitude of
heterosis in alfalfa and cotton was positively associated with genetic
diversity between the parents involved in the cross. ^ in .maize,
increased genetic differences between inbred lines results in a greater
heterosis in their hybrids. However, the maximum heterosis generally
occurs at an optimal or intermediate level of diversity. The D
technique has been used in. assessing the variability in crops like
maize, jowar, bajra, wheat, linseed, cotton, tobacco, alfalfa and
brassicas (Moll and Stuber, 1974),
k In addition to aiding in the selection of divergent parents for
hybridization, O 2 statistic measures the degree of diversification and
determines the relative proportion of each component character
to the total divergence.' The genotypes grouped together are
less divergent than the ones which are. placed in different,
clusters. The cl sisters which are, separated by the greatest statistical
distance show the maximum divergence. Three important points-
should' be taken into consideration while selecting parents on the
basis of D* statistic. These points are : (1) the relative contribution
of each character to the total divergence, (2) the choice of clusters
with' the maximum statistical distance, and (3) the selection of one
or two genotypes from such clusters. Other characters, like disease
resistance, earliness, quality, etc., should also be considered. - Cross-
ing of the selected lines In a- di aiiei fashion may generate some
useful segregants. : v ■'
X Metroglyph Analysis. This is a semigraphic method of studying
variability in a. large' number of germplasm lines. taken at^ a time.
This technique was developed by Anderson in 1-957 to investigate the
pattern of morphological variation in crop species, The. analysis of
variation is based on the mean values for. different traits. To begin
with, two characters exhibiting the highest variability are identified.
One of these traits, say, pod number in green gram, is used as the
O LOW ME&IUP1 HiaH
POX>S PER PlAMT
Fie 5 l A metroglyph representation of five quantitative traits in 32 lines
F 8 ' ' of green gram. The X-axis depicts pods plant, while the Y-axis
repreSms yield/plant. Each line is represented by a circle ; the
position of a circle on the graph is determined by the mean pods/
plant and yield per plant of the concerned hoe. The three rays
emanating from each circle represent the three remaining traits ,
the rays are either short, medium or long showing low, medium,
high respectively, mean values for the concerned trait tn the
retpective'* line, ^he X- and Y-axes are divided into low, medium
and high sections ; this divides the graph into nine sections,
character occupies a definite position on the glyph. The range of
variation in a character is represented by the length of the corres-
ponding ray. For convenience, the mean values for each trait are
classified into three groups, viz*, low (index score, 1), medium (score,
2), and high (score, 3). The length of each ray, as a result, is either
short (low mean value), medium (medium mean value) or long (high
mean value). The worth of an individual line is assessed from the
sum of index values for all the characters represented in the graph.
The maximum and the minimum scores that an individual line can
get will be 3« and n, respectively, where n is the total number of
characters studied
The X- and Y-axes are also demarcated into low, medium and
high mean values. This divides the entire graph into nine quad-
rangles, each quadrangle representing one variability group. The
variation is studied within a group as well^ as among . the groups.
This technique has been used for the analysis of variability in vari-
ous crops.
Biometrical Techniques in Plant Breeding
AIDS TO SELECTION
Selection is perhaps the most important activity in all plant
breeding programmes. The various types of selection schemes, viz. 9
mass selection, progeny selection, and cyclic selection, are used
■depending on the mode of pollination of the crop species, the pre-
dominant gene action, and the br ceding objective. Selection is
practised both in homozygous as well as segregating populations.
The efficiency of selection largely depends on the extent of genetic
variability present in the population, and the heritability of the
concerned character. Selection is generally more effective for charac»
ters with high heritability than those having low heritability.
Higher yields are the breeding objective in all the crops, but
generally yield has low heritability. Yield is regarded as a complex
character or super character, which is influenced by many compo-
nent or contributing traits both in positive and negative directions.
Generally, direct selection for yield is not sufficiently effective
due to its low heritability, and it is desirable to select indirectly
for improved yield. Some biometrical techniques provide informa-
tion about the relative contributions of the various component traits
to yield. Therefore, these techniques aid in the isolation from
populations of superior yielding genotypes by providing information
for indirect selection for yield. These techniques include, (1)
correlation coefficient, (2) path coefficient, and (3) discriminant
function analyses.
Correlation Coefficient Analysis. The statistics which measures the
relationship between two or more variables is known as correlation
coefficient . Correlation coefficient analysis measures the mutual
relationship between various plant characters and determines the
component characters on which selection can be based ' for improve-
ment in yield. Mass selection has been used to improve grain yield
through indirect selection for highly heritable traits which are asso-
ciated with yield. Correlation coefficients are of three types :
(!) simple or total, (2) partial, and (3) multiple correlations.
I. Simple Correlation. It is the association between any two vari-
ables. Simple correlations can be estimated from both unrepli-
cated as well replicated data using the following formulae.
In case of unreplicated data ,
x. y/(V*.
where, Co* x.y~[Zx.y - {(£x)(2j)/tf}]/(N - 1)
V^^-[Sx I. 2 --{(Sx) 2 /N}]/(N^I)
- {(2y) £ /N}]/( N-l)
2x.y«*$um of the products of all the observation®-
on the x and y variables,
2x *am of all the observations on the variable
IOC)
Plant Breeding : Principles and Methods
Sj— sum of all the observations on the variable y t
£* s ,- 'Ey —the sums of squares of all the observations
on x and y variables, respectively, and
N=the number of observations on x and y
variables.
Similarly, in the ease of replicated data ,
r~MSP*/(MS** MS*,) 1 ' 2
where, MSP* —The mean sum of products for treatments
(lines, varieties, strains) for the variables x
' and y 9
MS#*— Mean squares for treatments for the variable x 9 and
MS^==Mean squares for treatments for the variable y\
The estimates of MSP#, MS#* and MSq. are obtained from
analyses of covariance (for x and y variables) and variance (sepa-
rately for variables x and y) tables based on the replicated data.
The simple correlation is- of three types : (1) phenotypic,,
(2) genotypic, and (3) environmental. Phenotypic correlation is the ‘
observable correlation between two variables ; it includes both
genotypic and environmental effects. Genotypic correlation , on the
other hand, is the inherent association between two variables ; it
may be either due to pleiotropic action of genes, linkage or more
likely, both* Similarly, environmental correlation is entirely due to
environmental effects.
These three types of correlations can be estimated from repli-
cated data only. Following the analyses of variances and covart»
ance of such data, phenotypic, genotypic and environmental
variances and covariances are estimated in the same manner as
described for variances in Chapter 4, Mow, phenotypic, genotypic
■ and environmental correlation coefficients are estimated according-
to the following formulae.
r P Co, x . j/(PV„ PV y ) 1/2
r ff =G Co, x . y/(GV® . GV,) 1 ' 2 , and
r#-E CopX.y/(BV x . EV,) 1 ' 1
where, r, P9 r& and re are phenotypic, genotypic and environmental
respectively, correlation coefficients ; P Co, x . y 9 G Co, x y and
E Co» x . y are phenotypic, genotypic and environmental, respec-
tively, covariances between variables x and y ; PV„ GVpmd EV* are
phenotypic, genotypic and environmental, respectively, variances for
the variable x ; and PV y9 GV y and EVj, ■ are phenotypic,, genotypic
and environmental, respectively, variances for the variable y.
2. Partial Correlation. When the correlation -between two vari-
ables, say, Xr and X ? , is estimated by taking into account the
effect of a third variable, e.g. s X 3 , it is called partial or net ,
Biometrical Techniques in Plant Breeding 107 '
correlation, and is denoted as r i2 * 8 . It provides a better In-
sight Into the true relationship between the two variables X s
and Xg than Is available from the estimate of simple correlation
coefficient between them. It Is calculated from the estimate
of simple correlation coefficients according to the following
formula,
a * %—( ri 2 -r iz . n z )j{(l -r 13 2 )(l - r 23 2 )} 1/2
where* r 12 , r i3 and r 23 a, re the estimates of simple correlation
coefficients between the variables X x and X 2t X x and X 3j and Xg
and X 3) respectively.
3. Multiple Correlation. The estimate joint influence of two or
more variables on a dependent variable is called multiple
correlation. Such an estimate helps in understanding the
dependence of one variable, X l9 on a set of independent variables,
say, X a , X 3 etc. It is denoted as Ri- 2 s\ where, R 2 is the coefficient
of multiple correlation, i is the dependent variable X x , and « and
§ are the independent variables X 2 and X s . It measures the joint
Influence of the independent variables X s and X 3 on the dependent
variable Xi. The multiple correlation coefficient is estimated from
the estimates of simple correlation coefficient as follows.
R 2 i-23 === ('*x2 2 +ri3 2 —2r l8 . r n • r 2 s)Ki-N/)
The equate root R 8 3 . 23 is the estimate of multiple correlation
coefficient The coefficient of determination can be estimated as the
square of the coefficient of multiple correlation. Thus,
Coefficient of determination = R 2 i. 23
Fatli Analysis. The concept of path coefficient analysis was origi-
sally developed by Wright in 1921, but the technique was first used
for plant'Selection by Dewey and Lu in 1959. Path analysis is simply,
standardized partial regression coefficient which splits the correla-
tion coefficients into the measures of direct and indirect effects of a
set of independent variable? on the dependent variable. For example
in black gram, grain yield (X 5 ) is affected by the number of primary,
branches (X 3 ), secondary branches (X 2 ) pods per plant. (X 3 ) and seed
per pod (X 4 ). The path analysis unravels whether the association of
these characters with yield is due to their direct effect on yield, oris
a consequence of their indirect effect via some other trait(s). If the
correlation between yield and a character is due to the direct effect
of character, it reflects a true relationship between them and
selection can be practised for such a character in order to improve
yield. Rut if the correlation is mainly due to indirect effect of the
character through another component trait, the breeder has to select
for the latter trait through which the indirect effect is exerted.
Path analysis Is carried out using the estimates of correlation
coefficients. To begin with, all possible correlations among the de-
pendent and independent variables are worked out. Now the path
analysis is undertaken in three steps, viz., calculation of (1) direct
effects, (2) indirect effects and (3) the residual effect*
IQg Plant Breeding : Principles and Methods
L Direct*! Effect Every component character will have a direct
effect on yield. In addition, it will also exert indirect effect via other
component characters. In order to estimate the direct effects of
different component traits, first a path diagram is constructed as
follows (Fig. 5.2), using the estimates of correlation coefficients.
With the help of the path diagram, simultaneous equations are
•developed as follows.
•Fig. 5.2. A path diagram constructed using the correlation coefficients among
snri fnnr of its component traits in black gram. R denotes
yield (Y) and four of its component traits m black gram,
the residual effect.
^6=^2 s+ns * Fgs+ris . F 3 5 4 -r 14 . P45
7*25 —i r l2 • Fl5'h’F254" r2 3 * ^35 + ^4 » F 4 S
£ Fi54-fg 5 . F3 5 +F35+r S4 . F45, and
r 45 *«r 1 4 * Pi 5 +r 24 • Pgs+fs* . Pes+P^s
where, r i2 , ri 4 , etc. are the estimates of correlation coefficients
between variables Xi and X 2 , X t and X 8 , Xi and X 4 , etc., respec-
tively, and P 15 , P £5 , P35 and P 45 are the estimates of the direct
■effects of variables X x . X 3 , X 8 and X 4 , respectively, on the
dependent variable X5 (in this case yield). After putting the values
■of the correlation coefficients in these equations, the values of
P 15 * P25 etc. are estimated by the process of elimination.
2 . Indirect Effects. These effects are computed by putting the values
of correlation coefficients and those of direct effects as follows.
Indirect effect of primary branches (Xj) via,
secondary branches (X 2 ) =r,* . P 26
pods per plant (X 3 ) =?« . P S 5
seeds per pod (X*) — r M . ^45
Similarly, the indirect effects of secondary branches (X*) will be as
follows.
via primary branches (XJ =n 8 . P 3s
via pods per plant (X 3 ) ==r 23 P 35
via seeds per pod (X 4 ) rg 4 . ? 4 s
The Indirect effects of the other component traits, e.g. 9 pods per '
plant (X 3 ) and seeds per pod (X 4 ), may be computed in a similar"
fashion.
3. Residua! Effect:. Using the values of direct effects and com ■-
lation coefficients, the residual effects are estimated according-
to the foliowing formula, the equation is based on the earlier
example of black gram.
1— F 2 ^ s +Fi 5 * fis+Pss * r 25 -f Ps 5 . r M +P 46 . r 4 $
where, P a Rs is the square of residual effect.
Information obtained from path analysis has been extensively
used in different crops for Indirect selection for yield. For example
in oats,, indirect selection through seed width resulted in a 9%
increase in grain yield. Selection for a component trait with a view
to improve yield is called indirect selection , while selection for yield
per se is termed as direct selection . In maize, indirect mass selection
for number of ears per plant resulted in an increased yield. A
greater yield response is obtained when the character for which
indirect selection is practised has a high heritability and a high
correlation with yield. The minimum combinations of heritability
and correlation coefficient values necessary for indirect selection to
be more efficient than direct selection for yield itself have been
tabulated (see, Moll and Stuber, 1974).
Discriminant Function. The use of discriminant function for plant
selection was first proposed by Smith in 1936. He suggested that a
better way of exploiting genetic correlation with several traits hav-
ing high heritability is to construct an index, called selection index ,
which combines information on all the characters associated with the
dependent variable, yield. Selection index involves a selection crite-
rion based on a combination of measurements on various characters.
The best known selection indices involve discriminant functions
based on the relative economic importance of the various characters.
This technique provides information on yield components and thus
aids in indirect selection for the improvement of yield. Since the
desirable genotypes are discriminated from the undesirable ones,
based on the combinations of various characters, this technique is
known as discriminant function analysis. Later on in 1943, Hazel
developed a simultaneous selection model based on the approach of
path analysis ; subsequently several modifications were made in the
technique by different workers for specific breeding requirements.
It has been pointed out that there exists some uncertainty in
the usefulness of this technique due to the errors in estimation of”
the statistics upon which the index weights are based. In order to
solve some of the problems associated with the estimations, in 1957’
aid to the choice of parents and breeding
PROCEDURES
Hybridization is the most potent technique for breaking yield
barriers’and evolving varieties having a built in high yield potential.
The selection of suitable parents for hybridization is one of the most
important steps in a hybridization programme. Selection of the
parents on the basis of phenotypic performance alone is not a sound
procedure since phenotypicaliy superior lines may yield poor re-
combinants in the segregating generations. It is, therefore, essen-
tial that parents should be chosen on the basis of their genetic value.
There are several techniques for the evalution of varieties or strains
in terms of their genetic makeup. Of these, diallel, partial diallel
and line X tester techniques are in common use. A brief description
of these and some other techniques is given below.
Diallel Cross Analysis, Diallel cross refers to all possible crosses
SohTFlihes, _ arrd"'fHe analysis of such a set of crosses is known as
diallel analysis. Diallel analysis is based on the following seven
assumptions (Hayman, 1954).
1. Norma! diploid segregation.
2. Lack of maternal effects.
3. Absence of multiple alleles.
4. Homozygosity of parents.
5. Absence of linkage among genes affecting the character.
6. Lack of epistasis. ;
7. Random mating.
There are two approaches for diallel analysis : (1) Hayman’s
approach and (2) Griffing’s approach^
I Hayman’s Graphical Approach, this approach is based on the
estimation of components of variation. It was initially
developed by Jinks and Hayman in 1953, and later elaborated
n various traits are based on the
averaee statistics for several populations. An experimental compa-
rison of these two types of indices (as per ornith and according to
Hanson and Johnson) for soybean yields shows that only the specific
index as nrooosed by Smith was more effective than direct selection
for vield However, it is doubtful that indices are sufficiently more
effective ‘than the selection for yield alone in order to justify the
extra effort required in constructing them.
Selection indices are generally classified into three categories :
fl) classical, (2) restricted, and (3) general. The estimation or all
these indices is based on the estimates of phenotypic and genotypic
variances and covariances of the characters involved in the index
Worked examples of the three models are available m Singh and
Chaudhary (1985).
Biometrical Techniques in Plant Breeding HI
•independently by. Jinks and Hayman. The following six
components of variation are estimated.
D — additive genetic variance.
Hi — dominance variance.
Hi =* Hi{ \ ~(u~ v) 2 ], where, u and v are the proportions of
positive and negative genes, respectively, in the parents.
E = expected environmental component of variance.
i* = mean of Fr over the arrays, where, Fr is the covariance
of additive and dominance effects in a single array.
h 2 = dominance effect, as algebric sum over ail the * loci in
heterozygous phase in ail the crosses.
From these estimates, the following genetic ratios are deter-
mined.
1. Average degree of dominance (-HJ D) lli . If the value of this
ratio is zero, there is no dominance ; if it is greater than zero
but less 1, there is partial dominance ; if it is equal to !, there
is complete dominance ; and if it is greater than 1, it denotes
overdominance.
2. The ratios of dominant and recessive genes in the parents is
estimated as follows,
[(4 DH^F + F] j [(4DJf,)'/s _ p ]
If this ratio is 1, the dominant and recessive genes in the parents
are in equal proportion .; if it is less than 1, it indicates an excess
ot recessive genes , but if is it greater than one, an excess of
dominant genes is indicated.
3. The number of gene groups which control the character and
exhibit dominance is given by,
4. The proportion of genes with positive and negative effects in the
parents is estimated as.
If the positive and negative alleles are symmetrically distribut-
ed, this ratio equals 0.25.
The above components are estimated from the data on Fi
generation of a diallel cross. In 1956, Jinks gave the procedure for
estimating these components from F 8 data as well. The coefficients
of if, and are * in F„ while they are 1 in F,. The h and F Have
a coefficient of* because of the one generation of inbreeding
to obtain F a . Accordingly, the genetic ratios in F 2 are worked out
as follows.
1. Degree of dominance = / 2>)]V 2
2. Proportion of dominant and recessive genes in the parents
HliVDHi) 1 '* + \F}!{\ {4Z?j? 1 ) 1/2 — IF}]
th, ge ° es , u with Positive and negative effects in
me parents and the number of groups of -genes which
m
Plant Breeding : Principles and Methods
control the character and exhibit dominance are tbe same
in F$ as m Fj.
V r W T Graph. V r Is the variance of r m array and Wr is the
covariance between the parents and the offspring in the r iA array.
The relationship of F f - W f provides some useful information ; it
is generally depicted graphically (Fig- 5.3). Some of the inferences,
that can be draws from such a graph are as follows.
Fig. 5.3. Interpretations of the V r -~W r graph depending on the path traversed
by the regression line. The limiting parabola (dotted line) Joints
the points of the arrays whose common parents contain all the
dominant or all the recessive alleles.
1. When the regression line passes through the origin, it indicates
complete dominance (D~H X ).
2. When it passes above the origin, cutting the W r axis, it shows
that there is partial dominance (D>Hi).
3* When it passes above the origin, cutting the W f axis and
touching the limiting parabola, it suggests the absence of
dominance.
4. But when It passes below the origin, cutting the V t axis, it
denotes the presence of overdominance.
5. The position of parental points on the regression line
indicates the dominance order of the parents. The parents
with more dominant genes are located nearest to the origin,
while those with more recessive genes fall farther from the
origin. The parents with equal frequencies of dominant and
recessive genes occupy an intermediate position. ■
Biometrical Techniques in Plant Breeding
113
2, Grifiiag’s Numerical Approach. This approach is based on the
estimation of gca (general combining ability) and sea (specific
combining ability) variances and effects* Griffing (1956) has given
four different methods for dialfe! analysis depending on the material'
included in the experimentation as shown below (Table 5.1).
Table 5.1. The four afferent types of diallei sets; involving n parents, for
. which Griffiog (1956) has outlined the procedure for combining
ability analysis.
Materia i included in
Number of entries
the experiment
in ’ the experiment
(Ffs -1- parents )
Parents, FTs and reciprocals
Parents, and FTs (without reciprpcals)
/!(»! + 1)/2
FTs and reciprocals (without parents)
«(n— 1)
Fj’s (without parents and reciprocals)
«(n— 1)12
In this approach, gene action is deduced through the estimates
of gca (general combining ability) and sea (specific combining
ability) variances and effects. The gca component is primarily a
function of the additive genetic variance. But if epistasis is present,
gca will include the additive X additive interaction component as
well. On the other hand, sea variance is mainly a function of
dominance variance* but it would include all the three types of
epistatic interaction components, if epistasis is present.
Diailel analysis has been extensively used in both self- and
cross-pollinated species to understand the nature of gene action
involved in the expression of quantitative traits. The merits and
demerits of diailel analyses are summarised below.
Merits. It provides a sensitive approach to large scale studies of
quantitative characters. It yields reliable information on the compo-
nents of variance, and on gca and sea variances and effects. Thus
it helps in the selection of suitable parents for hybridization as s well
as in the choice of appropriate breeding procedures.
Demerits . This technique can test only a limited number of parents
at a time. This is because for every increment of I in the number
of parents (//) the number of crosses increases by 2n— 2 in the case
of a diailel set including the reciprocals.. Many scientists feel that
in self-pollinated crops only the combining ability analysis should
be carried out. In any case, all the assumptions of the diailel are
seldom fulfilled ; the assumptions of absence of linkage and
epistasis are generally not satisfied. Procedures for handling the
diailel analysis in the presence of epistasis have been proposed *
one simple approach is to eliminate the parent and its array that is
interacting. "
Partial Diailel Analysis. The concept of partial diailel was
developed by Kempthorne in 1957 and was further elaborated by
114
Plant Breeding : Principles and Methods
Kempthorne and Curnow in 1951, and by several otbers. It is
sometimes termed as fractional diallel. It may be. defined as a
number of sampled crosses per parent or per array id all possible
combi nations of a given set of parents. The total number of
sampled crosses .is nsl 2, where* n is the number of parents and s
is the number of sampled crosses per parent or array. The s should
be either greater than or equal to n/ 2. Clearly* both n and s can
neither be odd nor even. If n is odd, 5 should be even and vice-
versa.
1. Sampling Procedure. For sampling, first a constant K is worked
out as follows.
K**(n+l-s)/2 -
If* =10, and s=5, K will be =(10+1 — 5)/2=3. This means that
in each array, five crosses are to be made (y=5), and sampling is
to begin after 3 arrays, i.e. s from the 4th array, as depicted below
(Table 5.2). In this partial diallel, only 25 crosses [=w/2=(10x 5)/2
=25) will be included.
Table 5.2. The procedure for sampling for a partial diallel set when «=10
and 5—5 ; here, tf=(l0+l-5)/2=3. The crosses included in the
partial diallel are marked by x.
Parents Pi P 2 Ps P « Ps Pfi P? Pa Ps> P 10
Pi
X
X
X
X
X
Pa
X
X
X
X
X
Pa
X
X
X
X
X
P 4
X
X
X
X
Ps
X
X
X
P*
X
X
P 7
X
Ps
P 9
P10
With the partial diallel approach, a large number of parents
can be evaluated at the same time* Let us suppose, 20 parents are
involved ; in a partial diallel, only (20 x : 1 1)/2= 1 10 (here, «« 20,
£=11) have to be made, while in a complete diallel-set, including the
reciprocals, 20 (20- l)/2— 190 crosses must be made ; this is almost
twice as much work. The partial diallel analysis provides informa-
tion about gca and sea variances and gca effects, and on the compo-
nents of genetic variance (<r 2 A and a 2 D), but irfails to estimate sea
effects.
Trialfe! Analysis. A triallel cross is a three-way cross or a complex
cress involving three parents, viz., (AxB) C. The all possible three-
way crosses among n lines will be,
w c 3 =3 [ n ! / (* -3) ! (3 ! ) ]
Biometrical Techniques in Plant Breeding
115
_ , r*( «-l)(i»-2) (n -3) 1 1
L ( n — 3) ! (3x2x1) J
— [« («-l) («—2) ]/2
The concept of triallel analysis was developed by Rawlings and
Cockerham in 1962. This analysis provides information on epistatic
components of tbe variance (cr 2 AA, <r 2 AD, <r 2 DB) as well It also
yields information on the order in which the parents should be
crossed for obtaining superior segregants. It has. been shown that
the \ 7 ariance components in a series of three way crosses will exhibit
the following genetic components. ‘ ; £| : l
o 2 three-way crosses =3/4 & 2 A4 1/2 <J 2 D-f9/16 a 2 AA+
3/8 a 2 AD + l/4 a*DD+
Quadriallel Analysis. The concept of quadriallel analysis was deve-
loped by Rawlings and Cockerham in 1962. It is 'an analysis of
double-cross hybrids, which are the first generation progeny of a
cross between two unrelated Fi hybrids, e.g. 9 (A x B) (C X D), where,
A, B, C and D are the four parents, and Ax B and CxD are the 1
two unrelated Fi hybrids-involving these parents. Thus quadriallel i|v I
analysis requires two cropping seasons for generating the experi-
mental material (as is the case for triallel as well), and is more
demanding than the triallel analysis in terms of the number of crosses fSy- 1
that have to be effected. . "
In order to generate the material, the F/s among the parental
lines are produced in a diailel fashion : the Fi’s are then mated
according to the diailel scheme, with the restriction that no parent
should appear more than once in the same double-cross combina-
tion, to produce the double-cross hybrids. For example, if the num-
ber of parents is 6, there will be 15 [== (6 X 5)/2 ] different single-
cross hybrids among them (excluding the reciprocals). These
single- crosses when mated in a diailel fashion, subject’ to the restric-
tion mentioned above, will generate,
3 ( n c 4 )=3 [n !/(4 ! ) (n— 4) ! ] double crosses.
where, n is the number of the parents involved. If #==6, the number
of all possible double-cross combinations will be,
=3 [nl/ (4 ! ) (n — 4 ! )=3 [6 ! / (4 ! ) (6 -4) l J~45
This technique offers the same additional information over the
other schemes as does the triallel analysis. It provides more estimates
of the components genetic variance of epistatic nature and also yields
Information on the order effects of the combinations in double-cross
hybrids. Variance components in a series of double-crosses show
the following genetic variance.
<* % doub!e-crosses= 1 /2 <r 2 A+lj4 a 2 D+l/4 ® 2 AA +
l/8 c*AD+l/16 o 2 DD +
Line X Tester Analysis. Line X tester cross is a modified form of the
top-cross scheme proposed by Davis in 1962. The line x tester
j 16 plant Breeding : Principles and Methods
technique was developed by Kempthorne in 1957. It is a good
approach for screening the germplasm on the basis o f gca and ! jca
variances, and effects. The total number of crosses to be made u
equal to the product of the number of lines and the number of
testers included in the study. Thus,
total number of crosses =?2 » t
where, n is the number of lines and t is the number of testers If
there are 30 lines and 3 testers, the total number of crosses will be
90 (=30X3).
Several tvpes of testers have been suggested by different
authors. Some feel that (1) heterozygous testers are superior to
homozygous ones. (2) The best inbred line has a masking effect due
to its desirable dominant alleles ; therefore, it should not be used as
a tester. Thus in order to obtain the real expression of a line, low
performing testers should be used. (4) An inferior synthetic deve-
loped by crossing together poor lines may be used as a tester. Thus
a synthetic susceptible to lodging should be used to test the capacity
of lines to withstand lodging. In general, a tester should be poor in
the traits for which the lines are to be analysed. Further, the testers
should be highly adaptable to environmental fluctuations.
The line x tester technique has been extensively used in almost
all the major field crops to estimate gca and sea variances and
effects, and to understand the nature of gene action involved in the
expression of various quantitative traits. This technique measures
the gca and sea variances and effects and the genetic components oi
variance (o*A and <r‘ l D). It however, fails to detect and estimate the
epistatic variance.
Generations Mean Analysis. This analysis is based on six popula-
tions, F„ P 2 , F ls F*. Bi and B 2 . Hayman (1958) and Jinks end Jones
(1958) developed the six parameter model for the estimation or
various components of genetic variance. Hayman (1958) has given
the procedure for estimating the various gene effects as follows.
w=mean effect =F 2 ._
rf=additive effect=B! — Bi. _____
//^dominance effect==F 1 — 4F S — iPj — !P 2 +2B 1 -j-2B a
i=additive X additive gene interaction
=2Bi+2B s -4F 2
y=additive X dominance gene interaction
~Si-!P7-2s+iPa
/=dominanceX dominance gene interaction
=P 1 +P 2 +2F 2 + 4F 2 - 4 B x — 4B 2
where, P x , P 2 , F 1( F 2 , B* and §2 are the mean values for the character
jn the Pi, P 2 , Fi, F„ Bj and B 2 populations, respectively.
Biometrical Techniques in Plant Breeding 1 17
The variances for these gene effect estimates are obtained by
the foliowing formulae :
V/M=V F* ^
Vd—V Bi+V B 2
Yh—V Fi+16 V F»+i V Pi+i V P 2 +4 V B t
+4VB 2
Vi— 4 V Bi+4 V Ba+16 V F s
V/=V Bj+1/4 V P,+V Bj+I/4 V P 2
y/=V Pl+V P a +4 V ^+16 V F 2 + 1 6 V Bi+16 V B,
In the absence of nonallelic interactions, Jinks and Jones (1958)
used the following formulae for the estimation of m, d and h (com-
monly known as the three parameter model).
m=l/2 Pi+1/2 P 2 +4 P 2 — 2 Bi — 2 B 2
J=Y 2 1/2 Pi- 1/2 P 2
h = 6 Bj+6 Bjj— 8 F 2 -Fi - 3/2 P, - 3/2 P 2
The variances for these estimates are calculated as follows.
Vm— 1/4 V Pi+1/4_V P 2 +16 V P,+4 V Bj+4 V B a
V<J=I V Pi + i V P 2
Vh=36 V Bi+36 V B a +64 V F 2 +V F x +I V Pj+f V P 2
When F 3 is included and the backcrosses (B x and B 2 ) a re
absent, five parameters, viz., m, d, h, i and / can be estimated as
follows.
m=F 2
j=.|Pi~|P 2 _ _
(4 Fi+12_F 2 -16 F a )
j=Pi-F 2 +i (Pr-P 3 +h)-i t .
/=J (16 F 3 -24 P a +8 1+)
The variances for these estimates are computed as follows.
Vm=VF 2 _
vd=i (v Pi+v p*)
V/t-iV (16 V F,+3.44 V F a +256 V F 3 )
V/=V Pi+V F s + i (V Pi+V P 2 +V /;)+rV V /
v/=i (256 V F 3 + 576 V F 2 +64 V Fi)
Mather estimated three components of the variance in the
absence of epistasis as follows (see, Chapter 4).
Heritable fixable variance (Z>)--4 VF 2 -2 (VB X +VB 2 )
Heritable nonfixable variance (E) —4 ( VB X + VB 2 — VF« — VE)
• Non-heritable nonfixable variance (E)=(VPi+VP*-fVFi)/3
118
Plant Breeding S Principles and Methods
The genetic advance, heritability and inbreeding depression can
be calculated as given below.
Genetic advance <Gs)=[VG/(VP) ,/2 ] x k
where, A: is a constant based on selection intensity, VG=VF 2 - VE,
and VP=VFo. Hence,
Gs-[(VF 2 ~VF i )(VF 2 ) 5 /2 j X k
And heritability, in broad sense, is estimated as follows.
Heritability (in broad sense) = [(VF 2 — VF a )/VF 2 ] x 100
while, heritability (in narrow sense) =»(i D/VF 2 ) X 100 (Warner, 1952)
or =[D/(D+H FE)] X 100 (Mather, 1949)
Further, inbreeding depression “[(Fj — hWEjjxlOO
The above techniques for estimating the components of vari-
ance provide information about the predominant type of gene action
for the important traits of a crop species. This helps in deciding on
a suitable breeding procedure for the improvement of various quan-
titative traits of the species.
Biparental Cross Analysis. The concept of biparentaf mating was
originally developed by Comstock and Robinson in 1948 and 1952.
In this technique, plants are randomly selected in F 2 or a subsequent
generation of a cross between two purelines having contrasting per-
formance ; the selected plants are crossed according to a definite
scheme (see below). Biparental crosses include full-sib and half-sib
progenies in the mating programme. Two important genetic para-
meters, /.*?., additive (<rb4) and dominance (<? 2 D) genetic variance
are estimated from biparental crosses. However, this analysis does
not provide any information on the epistatic component. The
biparental cross analysis' is based on the following eight assumptions.
1. Random distribution of genotypes in relation to variation.
2. Random choice of plants for mating.
3. Regular diploid segregation.
4. Absence of maternal effects.
5. Lack of multiple allelism.
6. Absence of epistasis.
7. Absence of linkage.
8. Equal surrvival of all genotypes.
Three mating designs for biparental crosses, commonly known
as. North Carolina Designs /, II and III (NCD I, II and III, respec-
tively) have been proposed. A brief description of these designs is
given below.
1. North Carolina Design I. In this design, five plants are
randomly selected from an P 2 or a subsequent generation of a
cross. One of 'these plants is designated as male and crossed
with each of the remaining four plants, which are referred to as
females. The set of four full-sib families thus produced is
Biometrical Techniques m Plant Breeding
denoted as' a male group ; f our such male groups (16 female
groups) constitute one set. A female group consists of one full-
sib family produced by crossing one female plant with one
male plant. In this scheme, a female plant (or animal) is used
for only one mating, while each male is mated to four different
females. The number of females mated to each male may be
more than four and may vary from one male to the other, but
usually it is kept uniform for ease in statistical treatment. The
design separates the variance of progenies into two fractions, viz.,
(1) variance due to males (<r 2 m), which is equal to i&*A, and (2) vari-
ance due to females (o' 4 * * * 8 /), which equals l® 2 A+la®D.
2. Netrh Carolina Design II. According to NCD II, equal num-
ber of males (or plants so designated) and females (or plants
so designated) are randomly selected from an F 2 population,
and each male is crossed with each female. Thus the total
number of crosses produced will be mXf, where m is the
number of males, ./ is the ' number of females, and m==/. In
this design, both maternal and paternal half-sibs are produced.
This design separates the variance of progenies into the
following three fractions : (1) variance due to males (==laM),
(2) variance clue to females (~i<r 2 A) and (3) variance due to
males x females (= Jor 2 D),
3. North Carolina Design III. In this design (NCD III), several
plants are randomly selected from an F 2 population, and are desig-
nated as males. Each male plant is backcrossed . to both the
parents involved in the cross to produce pairs of backcrpss pro-
genies. This design separates the variance into two fractions,
namely, (1) variance among males (=Jcr 2 A), and (2) variance due to
males X females (=-| v 2 D).
Of the three North Carolina Designs, NCD III is the most use-
ful as it requires a relatively smaller experimental area, and does not
depend on any .assumption about gene frequencies. Further, the
analysis of variance Is not Influenced by the presence of maternal
effects. NCD I and NCD II require 10-12 and 2-4 times, respec-
tively, as much area as NCD III.
The randomly selected plants from an F 2 generation may also
be mated either in pairs or in a dialled fashion. But the information
available from these two mating schemes is rather meagre, and they
are not in common me.
4. Triple Test Cross Analysis. The concept of triple test cross
(TTC) analysis was developed by Kearsey and Jinks in 1968.
It refers to the crossing of randomly selected F a plants with ’
both the parents involved in the cross, and with their F x
hybrid. Thus It is an extension of the North. Caroline Design
III- of biparental matings. Triple test cross provides informa-
tion about the presence or absence of epistasis, in addition to
120 Plant Breeding : Principles and Methods
estimating c % A md ® 2 D. In this design, a random sample of n plants
from an F g is crossed as male parent with three testers (which
are used as females) ; two of the testers (Li and L 2 ) are the parental
inbred lines or purelines as in the NCD III, while the third
tester (L s ) is the F t produced by crossing these inbred testers.
If the value (Ei+Lj— 2 E 3 ) deviates significantly from zero,
the presence of epistasis is indicated, -where, L u Ei and L§
are the mean values of the progenies derived from matings with
the tester L 1? L 2 and L s> respectively.
In 1976, Ketala et ah proposed a somewhat similar approach
in which the testers L 1% L 2 and their hybrid (L 3 ) are crossed with a
number of k unrelated strains, instead of random individuals
selected from the F 2 population derived from the cross Lx XL*, as
suggested by Kearsey and links in 1968. This technique is similar
. to TTC, has a similar statistical treatment, and yields comparable
information.
STUDY OF VARIETAL ADAPTATION
Varietal adaptability to environmental fluctuations is impor-
tant for the stabilization of crop production both over regions and
years. Adaptability is the ability of a genotype to produce a rela-
tively narrow range of phenotypes in different environments. It is
the result of genetic homeostasis , which refers to the buffering
capacity of a genotype to environmental fluctuations. The perfor-
mance of a genotype mainly depends on the environmental inter-
action. However, in many case a linear relationship is ^ found
between performance of genotypes and the environmental conditions.
Estimation of phenotypic stability, which involves regression
analysis, has proved to be a valuable technique in the assessment
of this linear relationship between the responses of genotypes
and the environmental changes.
The first systematic approach to the analysis of phenotypic
stability was made by Finley and Wilkinson in 1963, They used two
parameters, namely, (1) mean performance over all the environ-
ments, and (2) regression of the performances in different environ-
ments over the respective environmental means. In this model, a
regression coefficient of ( i ) unity indicates average stability,
' (ii) greater than one means below average stability, and (Hi) less than
one means the genotype has a greater resistance to environmental
changes and possesses above average stability. Absolute phenotypic
stability, would be expressed by a regression coefficient of zero.
However, in deciding about the worth of a genotype, its mean per-
formance must be considered aiongwith its phenotypic stability.
Otherwise a variety which is the lowest yielding in all the environ-
ments will necessarily show a b value of less than one,' Generally,
a variety having b=> l- f and a high mean yield would be considered
as' the most widely adapted* while a b value of one and a low
mean yield (over the environments) would indicate a poorly adapted
genotype*
Biometrical Techniques in Plant Breeding
121
In 1966, Eberhart and Russel made further improvements in
this 'analysis by partitioning the genotype X environment interaction
of each variety inte two parts, viz., (1 j slope of the regression line,
and (2) deviations from the regression line. They defined a stable
variety as one with a regression coefficient of unity (6=1) and a
minimum deviation from the regression line (sd*=* 0). Using their
definition, a breeder would usually desire to develop a variety with
high mean yield, and meets the above requirements for stability.
The skeleton analysis of variance table based in this model is given
below (Table 5.3).
Table 5.3. Skeleton analysis of variance table for stability as per Eberhart and
Russel (1966)
where, g t e, and r represent the numbers of genotypes, environment and
replications, respectively.
Ill 1968, Perkins and Jinks proposed a different model for
stability analysis. In this model, regression of genotypes X environ-
ments is obtained on the environmental index. In the Eberhatt and
Russel model, on the other hand, the variety performance is regressed
on the mean of all varieties grown under a specific environmental.
In the Perkins and Jinks model, the genotype x environment. (GX E)
sum of squares is further divided into two parts, namely, (1) sum of
squares (ss) due to heterogeneity among regressions,, and (2) ss due
to the remainder. The First component of ss is the same as ss due.
to' GxE (linear), while the second one is the pooled ss due To
deviations in the Eberhart and Russel model. In this model also,
regression coefficient and the deviations from regression, are used as
the parameters of stability. The analysis of variance is represented
as follows (Table 5,4).
Table 5.4. The skeleton analysis of variance table based on the Perkins and
Jinks model.
Source of variation d.f.
Genotype (G) g~\
Environment (joint regression) \ '^-T
Genotype x environment (S’—!) (e—1)
Heterogeneity among regressions ^—1
Remainder (g-~l)(e~-2)
Hrror ge (r—1) ' ,
where, g, e and r are the numbers of genotypes, environments and replications,
respectively.
Source of Variation
d.f.
Genotypes
g-i
Environment (E) + Interaction (GxE)
g(e-l)
Environment (linear)
■1
GxE (linear)
*-1
Pooled deviations
g(e~ 2}
Genotype I
e-~2
Genotype 2
e—2
Genotype g
£—2
Fooled error
ge(r~~l)
122
Plant Breeding : Principles and Methods
One of the basic statistical objections to the above models of
stability is the improper choice of the sum of squares and the
degrees of freedom from which to subtract the regression compo-
nents. In 1971, Freeman and Perkins presented a better method of
partitioning the sum of squares as given below (Table 5.5).'
Tab!© 5.5. The skelton analysis of variance table based on Freeman and
Perkins, !97i.
Source of 'Variation
d.f.
Genotypes (G)
Environments (E)
• e-1 ’
Combined regression
i
Residual (1)
e-2
Interaction (G x E)
(£- l)(e-t)
Heterogeneity of regressions
£-1
Residual (2)
(*-»(«- 2)
Error (between replications)
ge (r— 1)
where* g % e and r are the numbers of genotypes* environments and replications,
respectively.
In the earlier models, the estimates of mean performance and
environmental index are not independent. Freeman and Perkins
proposed independent estimation of these two values. In their
model s the replications are divided info two groups : (I) one group
is used to measure the mean performance of varieties in the different
environments, while (2) the other group is used for the estimation
of environmental index. Another approach . for stability analysis
is the use of one or more genotypes as checks to assess the environ-
ments. Although these measures provide the desired independence
of the environmental and genetic effects, they entail* additional
experimental costs. In this model, the two residuals sums of squares
form the pooled deviations.
Comstock and Moll, in 1963, defined the environment of a
single organism as opposed to that of another growing at the same
time in almost the' same place as micro-environment. The environ-
ment associated with variables having large and easily recognised
effects was termed as macro* environment and may include differences
over years, locations, fertilizer levels, planting dates, irrigation
schedules
Allard and Bradshaw, In 1964, classified the environmental
variation into two types: (1) predictable and (2) unpredictable.
The predictable variations include all the permanent features of the
environment, such as, soil type, day-length, planting* dates, and all
other agronomic practices etc. On' the other hand, the unpredic-
table variations include the fluctuating features of the" environment,
the fluctuations of weather (rainfall and temperature) etc.
Therefore, planting of experiments over years and locations is less
Biometrical Techniques in Plant Breeding 123
pertinent than that over a range of agronomic practices, such as,
differences in fertilizers, irrigation, sowing dates etc., which are
much more pertinent measures of a crop environment.
It is generally agreed that the more stable genotypes' can
somehow adjust their phenotypic responses, to provide some measure
of uniformity in spite of environmental fluctuations. The buffering
ability of the segregating populations seems to be directly related
with the homeostatic responses of the parental lines. Therefore, it is
feasible to develop phenotypically stable high potential genotypes
by ordering homeostatic genotypes into a hybridization programme,
SUMMARY
Biometry consists of the application of stat istsctical concepts and
techniques to the study of biological problems. Biometrical genetics is that
branch of genetics which deals with the study of quantitative inheritance. The
various statistical procedures which are used in the study of biometrical
genetics are known as biometrical techniques . These techniques are useful to
the plant breeder in four principal ways, viz., (1) in the assessment of variabi-
lity, (2) in the selection of elite lines, (3) in the choice of parents and breeding
procedures, and (4) in the study of varietal adaptation.
Variability can be assessed in theie ways : (1) by simple statistical
measures, namely, range, mean, standard deviation, coefficient of variation
and standard error; (2) through D® statistic, and (3), with the help of metro-
glyph analysis. B a statistic measures the forces of differentiation at ictra-
and intercluster levels and determines the relative contribution of each compo-
nent trait to the total divergence. The clusters which are separated by the
largest statistical distance show the maximum divergence. Metrogiyph
analysis is a semigraphic method of studying variability in large number of
germplasm lines at the same time.
Studies on correlation, path and discriminant function analyses help in
the selection of elite types . Correlation measures the mutual relationship
between two or more variables. Correlation is of three types : (1) simple,
(2) partial and (3) multiple. Simple correlation is the association between two
variables, and- is of three types, viz., (!) phenotypic, (2) genotypic, and (3)
environmental. Partial correlation measures the association between ■ two
variables by eliminating the effect of a third variable. Multiple Correlation ,
on the oiher hand, measures the joint influence of two or ‘ more independent
variables on a_ single dependent variable. Path Analysis splits the correlation
coefficients into the components of direct and indirect effects. Thus it measures
the direct and indirect contribution of independent variables on the depen-
dent one. Discriminant function analysis involves the development of a
selection criterion based on a combination of various characters, and aids in
indirect selection for yield.
. Three important biometrical techniques, namely, (1) diallel, (2) partial
diallei, and (?) line x Jester analyses are' used for the selection of desirable
parents based on the estimates of gca and sea variances and effects. There
are several other techniques, viz., generations mean analysis, bipaiental cross,'
triple test cross, triallel and quadriallel analyses, which are used to estimate
the components of variance from different populations.
Stability reflects the suitability of a variety for a general cultivation
over a wide range of environments. Phenotypic stability is measured by four
different models given by, (I) Finley and Wilkinson in 1963, (2) Eberbart
and Russel in 1966, (3) Perkins and Jinks in 1968, and (4) Freeman and
Perkins in 1971. The Eberbart and Russel model is widely used as it is
relatively simple* yet quite informative
124 Plant Breeding : 'Principles and Methods
QUESTIONS
1. Describe briefly the various biometrical techniques which are used for
assessing the variability in a collection of germplasm.
2. Some biometrical techniques are useful in providing information for
indirect selection for yield. Describe these techniques in brief.
3. List and briefly describe the various techniques used for the selection of
parents for hybridization programmes. Discuss their merits and demerits,
4. Differentiate between ' the following : (i) dominance and epistasis,
(ii) correlation and path analysis, (iii) variability and diversity,
' (iv) adaptation and adaptability, (v) diallcl and partial dialle! designs.
5. Diflne oiparental cross. Describe the different designs used for
biparentai matings ; and discuss their merits and demerits.
6. Explain the different types of correlations estimated in breeding materials
and discuss their implications in plant breeding.
7. What do the following terms signify ? (i) triple test cross, (ii) trialiel
analysis, (iii) quadrialle! analysis, (iv) selection index, (v) biparentai
cross, (vi) line x tester analysis. (vii) V r - W r graph.
8. Describe briefly the various methods for estimating the different compo-
nents of variance.
9. List the various biometrical techniques in common use. Discuss the
practical implications of any two of them in plant breeding programmes.
JO. Define stability. Describe briefly the different methods used for estimating
the phenotypic stability and discuss their merits and demerits.
II. Write short notes on the following : (i) biometry, (ii) D 2 statistics,
(iii) correlation coefficient, (iv) path analysis, (v) components of
variance, (vi) metroglyph analysis, (vii) generations mean analysis,
(via) path analysis, (ix) stability analysis.
Suggested Farther Reading
Eberhart, S.A. and Russel, W.L. 1966. Stability parameters for comparing
varieties. Crop Sci. 6 : 34*40.
Freeman, G H, and Perkins, J.M, 1971. Environmental and genotype -
environmental components of variability. VIII. Relations between geno-
■ types grown in different environments and measures of these environments.
Heredity 27 : 1 5-23.
Griffing, B. 1956. Concepts of general and specific combining ability In
relation to diallel crossing system. Amt. J. Biol. Sci. 9 : 463-493.
Hayman, B.L 1954a. The theory and analysis of diallel crosses. Genetics 39 ;
789-809.
Hayman, B.L 1954b. The analysis of variance of a diallel, table. Biometrics
10 : 235-244.
Hayman, B.L 1957. Interaction, heterosis and diallel crosses. Genetics 42:
336-355. .
Hayman,' B.1. 1958. The separation of epistatic from additive and dominance
variation in generation means. Heredity 12 : 371-390.
Biometrical Techniques in Plant Breeding
Kearsey, M J. and Jinks, J.L. 1968. A general method of defecting additive
dominance and ephtatic variation for metrical traits. I, Theory. Heredity
23 : 403-409.
JCemptjhorne, O. 1957. Ad Introduction to Genetic Statistics. John Wiley &
Sons, Inc., New York.
Mather, K. and Jinks, J.L, 197L Biometrical Genetics. Chapman and
Hall Ltd., London.
Moll, R.H. and Stuber, C.W. 1974, Quantitative genetics- empirical results
relevant to plant breeding. Adv. Agron. 26 : 277-310.
Pollak, E., Uemphorne, O. and Bailey, Jr., T.B. ■ 1977. Pwc, lot. Coof.
Quantitative Genetics, Aug. 16-21, 1976; Iowa State University Press,
Ames. .
Singh, R.K. and Chaudhary, B.D. 1985. Biometrical Methods in Quanti-
tative Genetic Analysis. Kalyani Publishers, New Delhi,
CHAPTER 6
Selection in Self-Pollinated Crops
In self pollinated crops* selection .. permits reproauction only in
those plants that have the desirable characteristics. Tills is achieved by
raising the next generation from seeds produced by # the selected
plants only ; seeds from the remaining plants are rejected. Selec-
tion is essentially based on the phenotype of plants. Consequently, the
effectiveness of selection primarily depends upon the degree to which
the phenotype reflects the genotype. Selection has two .basic charac-
teristics or limitations. First , selection is -effective for heritable,
differences only ; its effectiveness is greatly affected by heritability of
the character under selection. Second , selection does not create new
variation ; it only utilizes the variation already present in a popula-
tion. Thus the two requirements of selection are t (/) variation must
be present in the population, and (2) the variation must be heritable .
The purpose of selection is to isolate desirable plant types from the
population. Thus selection is basic to crop improvement, indeed* it
is one of the two fundamental steps of any breeding programme;
the two basic steps are : creation of variation and 1 selection,
HISTORY OF SELECTION
Before domestication* crop species were subjected to natural
selection. The basis for natural selection was adaptation to the
prevailing environment. Plant tppes more adapted to the environ™
ment produced more progeny than -the others. After domestication,
man has knowingly or unknowingly practised some selection. Thus
crop species under domestication were- exposed’ to both natural and
artificial selection, *.<?., selection by. man. For a long period under
domestication, natural selection was perhaps more important than
selection by man. But in modern plant breeding methods, natural
selection is of little value, and the current breeding methods depend
entirely on artificial selection.
There is evidence that selection' was practised by farmers in
ancient times. During late eighteenth century, selection bv agricul-
Selection in Self Pollinated Crops
12?
turists, such as Van Mom in' Belgium, Andrew Knight in England
and Cooper in U.S.A., resulted in several important crop varieties.
Le Couteor, a fanner of the Isle of Jersey, published his results on
selection in wheat in the year 1843. He concluded that progenies
from single plants were more uniform than the remaining popula-
tion, and that different progenies were of different agricultural value.
About the same time, a Scotsman named Patrick Shireff practised
individual plant selection in wheat and oats, and developed some
valuable varieties.
Some years later, beginning in 1857, Hallet in England prac-
tised single plant selection in wheat, oats and barley. He believed
that acquired characters were inherited. From the best plants, he
selected the best spike from which he selected the best grain.
Although iHs now definitely known that acquired characters are not
inherited, Hallet developed several commercial varieties, e.g> s
Chevalier barley. About this time, Vilmorin proposed individual
plant selection based on progeny testing. This method successfully
Improved the sugar content in sugarbeets (Beta vulgaris) in 12 years,
but 50 years of selection was ineffective in improving four wheat
varieties. This clearly demostra ted the difference between effective-
ness of selection in self- and cross-pollinated species. But the genetic
basis for this difference was understood only later*
The Svalof experiment station of Swedish Seed Association,
established in 1866, refined the single plant selection or pureline selec-
tion to its present form. Subsequently, the ‘genetic basis of purelines
was explained by Johannsen in 1903.
THE PROGENY TEST -
Evaluation of the worth of plants on the basis of the performance
of their progenies is known as progeny test . The progeny test was
''developed by Louis de Vilmorin. Therefore, it is also known as the
Vilmorin Isolation Principle or, in short, Vilmorin Principle . Vilmorin
observed that sugarbeet plants with high sugar content could be
grouped into three classes based on the sugar content of their pro-
genies. .The first group of plants produced progenies high in sugar
content, the progenies from the second group had some plants with
high and some with low sugar content, while the third group pro-
duced progenies low in ■ sugar content. Thus plants similar in
phenotype (high sugar content) produced considerably different
progenies. From these observations, Vilmorin concluded that the
real value of a plant can be known only by studying the progeny pro-
duced by it. .
Today, progeny test is the basic step in every breeding' method.
The progeny test serves two valuable functions. Its first function is
to determine the breeding behaviour of a plant, i,e. f whether it is
homozygous of heterozygous. For example, consider the progeny
128
Plant Breeding : Principles and Methods
from- barley plants with the genotypes VV and Vv, both having two-
rowed spikes. The second ( and perhaps the more important, function
of progeny test is to •' find out whether the character for which the
plant was selected is heritable, i.e., is doe to genotype. Selections
have to be based on phenotype* The relative contributions of geno-
type and environment to the phenotype of the selected plants can be
determined through progeny test. If the phenotypic differences are
due to differences in genotype, .they will be present in the progeny as
well ;■ otherwise they will be absent in the progeny. This will become
more clear when we examine the pureline theory of Johannsen*
THE PURELINE THEORY
A pureline is progeny of a single homozygous plant of a self-
pollinated species . Therefore, all the .plants in a pureline have the
same genotype. The phenotypic differences within a pureline are due
to the environment and have no genetic basis. Therefore, variation
within a pureline is not heritable.
The concept of pureiines was proposed by Johaxmsen in 1903 •
on the basis of his studies with beans (Phaseolus vulgaris). Beans are
strictly self-pollinated species. Johannsen obtained commercial seed
of the Princes variety of beans. The commercial seed lot showed
variation for seed size. ' He selected seeds of different sizes and grew
them separately. The progenies thus obtained differed in seed size :
progenies from larger seeds' generally produced larger seeds than
those obtained from ‘smaller seeds. This dearly slowed that the
variation in seed size in the commercial seed lot of Princess had a
genetic basis. As a result, selection for seed size was effective.
Johannsen further studied 19 lines; each line was progeny of a
single seed from the. original seed lot. He discovered that each line
showed a characteristic mean seed weight, ranging from 640 mg in
Line No. 1 to 350 mg in Line No. 19 (Fig. 6.1). The seed size within
a line showed some, variation, which was much smaller than that
in the original commercial seed lot. Johannsen postulated that the
original seed lot was a mixture of pureiines . Thus each of the 19
lines represented a pureline, and the variation in seed size within each
of. the pureiines had no genetic basis and was entirely due to environ-
ment- - .
Confirmatory evidence was obtained in three ways. In the
first case, he classified the seeds from • each pureline into 100 mg
classes, and grew them separately. The mean seed weights of pro-
genies from different seed weight classes of a single pureline were
comparable with each other, and. with that of the parent pureline.
For example. Line- No. 13 had seed size classes of 200, 300, 400 and
500 mg. The mean seed weights of the progenies derived from these
seed weight classes were 475, 450, 451 and 458 mg, respectively
(Fig. 6.1).
Selection in Self-Pollinated Crops
129
PRINCESS BEAN
{Commercial seed iot>
V!
LINES FROM INDIVIDUAL SEEDS.
MEAN SEED
WEIGHT (mg)
l\
SELECTION
FOR LARGEST SELECTION
AND SMALLEST f \
SEEDS .
LARGEST
SEEP
\
smallest
I SEED
)3 7
I I
\\°v\ 350
\ V'\\
SEED WEIGHT CLASSES
\ \ N \
200 300 400 500
Individual seeds
of different sizes
selected to raise
individual seed
progenies
Seeds classified
into 100 mg
classes
SELEC HON FOR f I
SEED Sl2e largest smallest
CONTINUED SEED
FOR SIX |
GENERATIONS f
SELECTION
Icontinusd!
mean SEEO
WEIGHT (mg)
Fig* 6.1 ,
. PROGENIES
I I I I
475 4 50 451 458
Mean seed weight
of the- progenies
(mg)
Selection for seed weigiat In a commercial seed lot of the Princess
variety of be.an by Joharmsen.
The second line of evidence came from selection within a pure-
line* From each pureline, largest and smallest seeds were selected
to raise the next generation. . In the subsequent generations, large
seeds were selected in the progenies obtained from large seeds, while
in those from small seeds selection was done for small seeds. Six
generations of selection was ineffective in increasing or decreasing the
seed size. For example, after six generations of selection, the mean
seed weight in Line No. 1 was., 690 and 680 mg in the progenies
selected for small and large seeds, respectively (Fig. 6.1). Thus
selection within- a pureline was ineffective.
The third approach was to estimate parent-offspring correla-
tion./ The value of parent-offspring correlation within Line No. 13
■N,
/•
4, v
X’“ 4
130
Plant Breeding : Principles and Methods
was -~0.0!8±0‘038, that is, zero, while it was 0.336±p.008 in
the original seed lot of the Princess which is highly significant. The
parent-offspring correlation will be zero when the variation is non-
heritable, while it will be significantly greater than zero when the
variation has a genetic basis, i.e., is heritable.
These observations reveal that the variation for seed size in
the original seed lot of Princess had a genetic basis and was
heritable. But the variation within the purelines obtained from the
single seeds selected from this seed lot was purely due to the environ-
ment and, therefore, nonheritable. The two main conclusions from
the Jobannsens’ experiment are,
L A self-fertilized population consists of a mixture of several
homozygous genotypes. Variation in such a population has a
genetic component, and therefore selection is effective.
2. Each individual plant progeny selected from a self-fertilized
population consists of homozygous plants of identical genotype.
Such a progeny is known as pureline. The variation within a
pureline is purely environmental and, as a result, selection
within a pureline is ineffective.
EFFECTS OF SELF-POLLINATION ON GENOTYPE
Self-pollination increases homozygosity with a corresponding
decrease in heterozygosity . Inbreeding also increases homozygosity
and reduces heterozygosity. Inbreeding is' mating between individuals
related by descent, that is, having a common parent or parents.
Some examples of inbreeding are : sib mating (brother-sister matingX
half-sib mating (brother-stepsister mating) etc. Self-pollination is
the most intense form of inbreeding, since in this case the same indi-
vidual functions as the male as well as the female parent.
The effect of self-fertilization on homozygosity and heterozy-
gosity may be illustrated by an example. Suppose an individual
heterozygous for a single gene (Aa) is self-pollinated in successive
generations. Every generation of self-pollination will reduce the fre-
quency of the heterozygote Aa to 50 per cent of that in the previous
generation. There is a corresponding increase in the frequency of
the homozygotes A A and aa. As a result, after 10 generations of
selling, virtually all the plants in the population would be homo-
zygous, i.e., A A and aa. On the other hand, the frequency of
the heterozygote Aa would be only 0.095 per cent, which is negligible
(Table 6.1). It is assumed here that the three genotypes AA,
aa have equal survival. If there is unequal. survival, it may increase
or decrease the rate at which homozygosity is achieved. If Aa is
favoured, the rate of increase in homozygosity would be lower than
expected. But if Aa is selected against, homozygosity would in-
crease at a faster rate than expected.
Selection in Self-Pollinated Crops
131
Table. 6.1. Effect of self-fertilization on the frequency of homozygotes with res-
pect to a single locus Aa. This also gives the degree of homozygosity
with respect to any number of genes.
Number of gene -
Frequency (%)
Frequency (%)
rations of self-
fertilization 'AA
Aa
aa Homozygotes Heterozygotes
0 or homozygo- (or hetero-
si ty) zygosity )
0
0
100
0
0
300
1
25
50
25
50
50
2
(25+12.5)
25
(25 4-12.5)
75
25
3
(37.5+6.25)
12.5
(37.546.25)
87.5
12.50
4
(43.75+3.125)
6.25
(43.7543.125)
93.73
6.25
S
(46.875+1.562)
3.125
(46.8754-1.562)
96.874
3.125
6
(48.437 + 0.781)
1.562
(48.437+0.781)
98.436
1.562
7
(49.218 +0.390)
0.781
(49.218+0 390)
99.216
0.781
8
(49.608 + 0.195)
0.390
(49.608+0.195)
99.606
0.390
9
(49.803+0.097)
0.195
(49.803 + 0.097)
99.800
0.195
10
(49.900 + 0.048)
=49.948
0.097
(49.900 +0 048)
=49-948
99.896
0.097
n
(2*- */2" +1 )
<2/2* +s )
(2 :*-‘/2* +1 )
2(2"“ 1 )
2"* 1
or
(2"~72*)
2
2"* 1
(1/2")
When a number of genes are segregating together, each gene
would become homozygous at the same rate as Aa. Thus the number
of genes segregating does not affect the percentage of homozygosity.
The term homozygosity denotes the frequency of genes in homo-
zygous condition in the population. Similarly, linkage between
genes does, not affect the percentage of homozygosity in the
population*
Another - way of visualising the effect of self-pollination is to
. consider the frequency of plants which are homozygous for all the
genes* In case of a single gene, the frequency of completely homozy-
gous plants in a generation is the same as - the proportion of
homozygosity. But when two or more genes are segregating, the
proportion of homozygosity increases at a much faster rate than that
of completely homozygous plants. The proportion of completely
homozygous plants is given by the following formula.
Proportion of. completely homozygous piants=[(2 w —l)/2 m ] n
where, m is the number of generations of self-pollination and n is the
number of genes segregating. The porportion of completely homo-
zygous plants under self-pollination is given in Fig. 6.2, It is " clear
that as the number of genes increases, the proportion of completely
homozygous plants after a given number of generations of selling
decreases. But the effect of selling is so strong that even if 100 genes
132 Plant Breeding : Principles and Methods
are* segregating, more than 95 per cent of the population would be
coi|ipeIetely homozygous after only 12 generations. Linkage would
increase the proportion of homozygous plants since it would reduce
the number of genes segregating independently. Unequal survival
of different genotypes would have the same effect as in the case of
ho.mozygosity.
Fig, 6.2. Per cent of completely homozygous individuals after different gene-
rations of self-pollination ; the curves represent values when 1, 2, 5,
10, 20 and 100 geoes are segregating.
Thus, self-pollination has two main effects on the population :
first, all the plants in the population become completely homozygous,
and, second, the population is a mixture of several homozygous geno-
types. : : v;
ORIGIN OF VARIATION IN PURELINES
It is generally accepted that purelines show genetic variation
after some time. For- example, barley variety Atlas was derived
through single plant selection, Later, individual plant progenies
.were selected from Atlas. These progenies differed from each other
with .respect to quantitative as well as -some minor' qualitative
characters. The variation in purelines may arise from machanical
mixture, natural hybridization., chromosomal aberrations , and
mutations. ~
Selection in Self Pollinated Crops
133
Mechanical Mixture. During cultivation, harvesting, threshing and
storage, other genotypes may get mixed with a purefine. Such con-
taminations are quite common and may be avoided by careful hand-
ling. Often mass selection is practised to maintain the purity of
purelines.
Natural Hybridization. In most self-pollinated species, a low amount
of cross-pollination does occur, at least under some environmental
conditions. Thus cross-pollination may occur with other genotypes
grown nearby. Subsequent selling of the hybrids would produce a
number of new genotypes- Inmost self-pollinated species, natural
hybridization can be avoided by isolating the purefine from other
genotypes with a couple of rows of the same pureline, that is, two to
three rows of the pureline. on all the sides of the plot are rejected as
border rows.
Chromosomal Aberrations. Varieties of several crops, c.g., wheat,
oats and barley etc., show a small frequency of meiotic anomalies,
such as, chromatin bridges and chromosome fragments. These
aberrations may lead to duplication and deficiency for small chromo-
some segments. Generally, the effects of small deletions and dupli-
cations would not be distinguishable from point mutations or gene
mutations. It is, therefore, not definite as to what degree such
aberrations are responsible for the origin of variation in purelines.
Mutation. Mutation is a sudden heritable ' change , and generally
denotes a chemical change in a gene , that is , point mutation or gene
mutation . In practice, however, it is difficult to distinguish point
mutations from minute chromosomal aberrations, such as, duplication
and deficiency. It may be generalised that almost 11 the variability
in plant species lias originated due to gene mutations. Mutations
occur spontaneously, they are random, recurrent and usually specific
in their effect on phenotype.
Stadler in 1942 reported the spontaneous mutation rates for
certain genes in maize. The mutation rates varied greatly from one
gene to another. For example, gene R (produces colour) had a
mutation rate of 4.92 X 10~ 4 , and Su (nonsugary endosperm) had a
rate of 2.4 x KT 8 , while Wx (nonwaxy endosperm) did not show the
mutant allelic (wx, waxy endosperm) in the 1,503,744 gametes tested.
Thus the spontaneous mutation rate for oligogenes may vary from one
per 10,000 (i.e. 9 10~ 4 ) to one per' 1 9 000,000 (i.e., I0~ 6 ) gametes .
The frequency of mutations affecting quantitative characters is
not known. This is primarily due to the difficulty in distinguishing
the phenotypic effects of mutant allele from those of other genes
and the environment. However, there is evidence to suggest that
mutations affecting quantitative characters may be more frequent
than those affecting qualitative characters.
In 1935 and 1936, East published his observations on some
homozygous lines of Nicotiana rustica. These lines were obtained
from plants that arose parthenogenetically during interspecific
Plant Breeding : Principles and Methods
J34
hybridization. These lines were much more uniform' than inbred lines
produced by more than 10 generations of selling. After four genera-
tions of self-pollination, the homozygyous lines became as variable
as the inbreds. Mechanical mixture and natural hybrizadition were
carefully avoided during the period of 'study. Therefore, the varia-
tion in these lines must have arisen from gene mutations. . This
suggests that mutations affecting quantitative characters are quite
frequent, but generally they are difficult to detect.
. One may then enquire about the possibility of improvement
through spontaneous mutations occurring in purelines. The possi-
bility of such an improvement is relatively small for two reasons.-
The first reason relates to the recurrence of mutations. Since muta-
tions are recurrent, most of the mutations must have occurred in the
past Natural and artificial selection would have 'accumulated most
of the superior alleles thus generated. Therefore;,, a new mutation is
most likely to be inferior to the existing allele. The second reason
arises from the frequency of mutations. The spontaneous mutation
frequency is too low (1(T 5 to 1(T 8 ) to create enough variability in
few (3 or 4) generations to permit effective selection. But there are
some instances of improvement of a pureline through spontaneous
mutations, although of oligogenes. A spontaneous semidwarf
mutant of a rice variety Kalimoonch 64 has been released for culti-
vation as a new variety Shyama. The dwarfing gene of Shyama is
different from that of Dee-geo-woo-gen, the source of dwarfing gene
in most of the semidwarf varieties of rice in India.
GENETIC ADVANCE UNDER SELECTION
Selection, of necessity, is made on the basis of phenotype, and
phenotype is produced by the joint action of genotype and environ-
ment. Therefore, the phenotypic superiority of selected . plants
or families over the original population is not solely due to their
genotypic superiority. Improvement in ike mem genotypic value of
the selected families over the base population is known as genetic
advance under selection . Genetic advance under selection depends
upon (1) the genetic variability among different plants or families in
in the base population,' (2) the heritability of the character under
selection, and (3) the intensity of selection, i.e., the proportion of
plants or families selected. Genetic advance under selection may be
calculated as follows.
Gs**(k) (op) (H)
where, Gs is genetic advance under selection, k is selection different
rial, op is the phenotypic standard deviation of the base population
(the population which is being subjected to selection), and H is the
heritability of character under selection. The formula for genetic
advance under selection may be rewritten as,
Gj=(fc) (Vh) (VJVJ
The selection differential, k, is based on the mean phenotypic
values of the selected lines and of the base population, the pheno-
Selection in Seif-Pollinated CroPs
typic standard deviation"^)* and the intensity of selection, that is,
the proportion of population selected. The k is expressed in terms
of standard deviation units. Therefore, the value of k varies with
the intensity of selection only (Table 6.2).
Table 6.2. Values of selection differential, k, for different selection intensities.
Selection intensity in per cent Value ofk
(qln)*XlO0
1
2
5
to
20
30
*q is the number of selected plants or lines, and n ia the total number of
plants or lines in the original population.
The estimation of genetic advance under selection may be
illustrated by using the data given in Table 4.6. The heritability is
<0.914 and the phenotypic standard deviation is 1.92. Let us suppose
that the intensity of selection is 5 per cent, that is, the extreme 5 per
cent of the population will be saved to raise the next generation. The
value of k, therefore, is 2.06. The genetic advance under selection
would be, ■
- Gs*=*(k) (d 9 ) (H)
=2.06 X 1.92X0,914 mg/plant
^3.6 mg nitrogen/plant
It would be noted that the estimate of Gs has the same unit as the
concerned mean. Gs may also be expressed as per cent of the
population mean.
&s{m per cent of population mean)~(Gsl X) x 100
where, X is the mean of the base population.
EXPECTED GENETIC ADVANCE IN SEGREGATING
POPULATIONS
The expected genetic gain as calculated by the foregoing for-
mula is applicable to a mixture of pureliaes or clones, hot it is not
applicable to segregating generations. This is because purelines or
clones give rise to progeny which are identical is genotype $wth the
parent plant or family. Therefore, the genotypic value of pro-
geny remains the same as that of the parent plant or family. But
in .segregating generations, the selected plants are likely to be
heterozygous for few or several genes. Therefore, in the progeny new
gene combinations would arise du® to recombination. The genotypic
value of new gene combinations is likely to differ from that of
136
Plant Breeding J Principles and Methods
the parent plant. This may be expected doe to dominance and
epistasic components of genetic variation, which depend on hetero-
zygosity (except for the / component, le. 9 additive X additive inter-
action). Therefore, when dealing with segregating populations,
heritability in narrow sense (le. 9 //—additive ' variance/phenotypic
variance) is more appropriate for estimating G$. The use of broad
sense heritability estimates would give higher estimates of Gs than
would be practically realised. In this case Gs may be estimated as
follows.
Gs—(k) (op) (H, narrow sense).
Using the data on days to flowering in bajra (Chapter 4), we may
estimate Gs as follows. The H (ns) is 0.394, and ap of the .F 2 ,
the segregating generation, is 4.79. Let as assume that 5 per cent of
the extreme individuals will be saved. The Gs in this case would be,
Gs=(2M) (4.79) (0.394) days
«3.9 days
SUMMARY
Selection consists of permitting the reproduction of some plants and stopp-
ing the others from reproducing. Selection appears to be as old as cultivation
of plants. Vilmorin developed the progeny test. Ic progeny test , she value of a
plant is judged from the performance of its progeny. Progeny test is the basis
of modem plant breeding methods. In 1903, Johannsen put forward the pure-
line theory. A punellne is a progeny of a single, homozygous, self-pollinated
plant. All the plants within a pure! ins have the same genotype. The variation
among pureiines has a genetic component, while that within a pure! me is
purely environmental. Genetic variation within a pureiine may arise by mecha-
nical mixture, natural hybridization, chromosomal aberrations and gene
mutations. Mechanical mixtures and natural hybridization can be easily
controlled. But mutations and chromosomal aberrations are beyond human
control. Mutations affecting quantitative characters are known. The spontane*
ous mutation frequency for qualitative characters varies from !0~ 3 4 to 1Q“ S .
Self-pollination increases the frequency of homozygotes and the degree
of homozygosity in the population. The population becomes homozygous
rapidly, and finally it consists of a mixture of pureiines. The decrease in
heterozygosity for a single gene is by 50 per cent of that in the previous
generation. The proportion of completely homozygous plants is given by
l(2 m ~~l)l2 m ]\
The difference between the genotypic values of the selected plants and of
the original population is known as genetic advance under selection . In case of
mixtures of pureiines, the estimated genetic advance is a reliable estimate of
actual genetic gain. But in the case of segregating populations, the estimates of
genetic gain are often higher than the actual gains,
QUESTIONS
1. Write short notes on the following : (i) Vilmorin isolation principle,
(ii) Pureiine theory, and (lii) Genetic advance under selection.
2. Explain the pureiine theory of Johannsen. Discuss the sources of genetic
variation in pureiines.
3. Explain genetic advance under .selection. How does, expected genetic
advance under selection in a mixture of pureiines differ from that in
segregating populations ?
Selection in Self-Pollinated Cropi 137
4. Give a brief account of she history of selection in self- pollinated popula-
tions. Describe progeny test and discuss its usefulness in plant breeding.
5. Discuss the effects of self-pollination on genotype and explain Its Implica-
tions in breeding of self- pollinated crops.
6. Briefly describe the contributions of the following scientists*
*»$
(i) Vilmorin (ii) Johannsen
(iii I Le Couteur (iv) Mallet
Suggested Further Reading
Allard, R.W. 1960. Principles of Plant Breeding. John Wiley and Sons* Inc.,
New York.
Allard, R.W., Iain, S.K. and Workman, P.L. 1968. The genetics of in-
breeding populations. Adv. Genet. 14 : 55- 1 31.
Johannsen, W.L. 1903. Ueber Erblichkdt in populationen und in reman .
Leinen. Gustav Fischer, Jena.
CHAPTER 7
Hybridization : Techniques and.
Consequences
Natural variability present in self-pollinated populations Is
lost quickly when they are subjected to selection. Individual plant
selection or pureline selection Is the most common procedure
used for the improvement of self-pollinated crops. Consequently, the
variability is soon exhausted as the land varieties are replaced
by purelines. For further improvement, therefore, new genetic
variability has to be created by plant breeder. This is easily and
most commonly achieved by crossing two different purelioes.
The mating or crossing of two plants or lines of dissimilar
genotype is known as hybridization. In plants, .crossing is done by
placing pollen grains from one genotype, the male parent > on to the
stigma of .flowers of the other genotype, th c female parent. It is
essential to prevent self-pollination as well as chance cross-pollinu-
tion in the flowers of the female parent. At the same time, it must
be ensured that the pollen from desired male parent reaches
the stigma of female flowers for successful fertilization. The
seeds as- -well as the progeny resulting from the hybridization are
known as hybrid or Fu The progeny of F lf obtained by selling
or intermating of F % plants, and the subsequent generations are
termed as segregating generations . The term cross is often used to
denote the products of hybridization, Le. y the F % as well as the
segregating generations. ' Ttv
HISTORY OF HYBRIDIZATION
In comparison to selection, hybridization is of more recent
origin* There is evidence that Babylonians and Assyrians hand-
pollinated date plains as early as 700 B.C. for mctaxcnic effects of
pollen. Clearly, the artificial pollination in this case was not for the
purposes of crop improvement. Metaxenia is the effect of pollen on
the maternal tissue of fruit. Sex in plants was discovered by
Hybridization : Techniques and Consequences 139
ss “& sa:
studies as well as for crop improvement. Notable among them are,
incf-nh Koelreutcr who made many crosses irs tobacco during 176 -
nS P a^mpbaS hybrid vigour in F t ; Thomas Andew Knight
who developed several varieties of apples, pears, peaches grapes
2?£S?fcto, 1759-1835 ; and Go,,. Sarga,et. 0»*.
Naudin had noted uniformity of F„ dominance »1.M
tion and appearance of parental types in F s . _ But it was lett
Mendel f 1865) to propose the clear-cut laws oi inheritance. These
and subsequent discoveries in genetics have given hyonumition a
scientific basis. Towards the end of nineteenth
lion was widely used for crop improvement. ¥»ith the help i t
cenetic principles, the breeder is able to predict the progeny he is
e? to obtain from a given cross. Therefore , he can plan the
crosses from which he is most likely to obtain the desired plant type.
Today, hybridization is the most common method o? crop tn’P^e-
ment,’ arid the vast majority of crop varieties have resulted fro
hybridization.
OBJECTIVES OF HYBRIDIZATION
The chief objective of hybridization is to create genetic varia-
fan When two genotypically different plants are crossed, toe genes
from both the parents are brought together in F 1; Segregation
and rcombination produce many new gene combinations in H and
the later generations, i.e., the segregating generations.. The degru..
of variation produced in the segregating generations would, there-
fore, depend on the number of heterozygous genes in the F t . -This,
will in turn, depend upon the number of the genes for which the
two 9 parents differ. If the two parents are closely related, they are
likely to differ for a few genes only. But if they are not related, or
are distantly related, they may differ for several, even a few hundred,
genes.' * However, it is not likely that the two parents wilt ever differ
for all the genes. Therefore, when it is said that the Fi is 100 p«r
cent heterozygous, it has reference only to those genes for which the
two parents differ.
The aim of hybridization may be transfer of one or few qualita-
tive characters, improvement of one or more quantitative characters,
or use of the Fi as a hybrid variety. These objectives are briefly
discussed below. _ .
Combination Breeding. The main aim of combination breeding is
the transfer of one or more characters into a single variety from other
varieties / These characters may be governed by oiigogenes or
polygenes. The intensity of the character in the new variety is either
comparable to or, more generally, lower than that m the parent
variety from which it was transferred. In this approach, increase m
the yield of a variety is obtained by correcting the weaknesses in the
* 40 Plant Breeding : Principles and Methods
traits ’ e ‘ g ;‘ tiiier number « grains per spike, test
weight etc of the concerned variety. A familiar example of com-
bination breeding is that for disease resistance. \ The backcross
method of breeding was designed for combination 5 breeding and
hre e ,H- P t ’i ree metllod also the same purpose. In combination
breeding, the genetic divergence between parents is not the major
consideration. _ What is important is that one of the parents must
have m a sufficient intensity the characters) under transfer, while the
other parent is generally a popular variety. ne
Transgressive Breeding. Transgressive breeding aims at improving
yield or its contributing characters through transgressive segregation
Transgressive segregation is the production of plants in an F
generation that are superior to both the parents for one or more
characters. Such plants are produced by an accumulation of plus
or favourable genes from both the parents as a conseqence of
recombination. Ooviously, the parents involved in hybridization
must combine well with each other, and should preferably be
genetically diverse i.e., quite different. This way, each parent is
contribute different plus genes which when brought
together by recombination give rise to transgressive segregants. As
a result, theintensity of character in the transgressive segregant / <?
the new variety, is greater than that in either of the parents, ’fbe
pedigree method of breeding and its modifications, particularly the
ZegTts. aPPr ° aCh ’ 3re dCSigned f ° r the production of transgressive
Hybrid Varieties. In most self-pollinated crops, F t is more vigorous
feasihl^F yie d) h g tha 2 pa r en£s ‘ Wherever it is commercially
feasible, may be used directly as a variety. In such case® it is
important that the two parents should produce an outstanding
TYPES OF HYBRIDIZATION
The plants or lines involved in hybrdization may belong to the
dlfferent pieties of the same species, different species
. of tile same genus or species from different genera. Based on the
taxonomic relationsphips of the two parents, hybridization may be
bybridzation° tW ° br ° ad gr ° UpS : 0) iatervarietal ap <* (2) “Sant
IflZT Hybridization - , The P arents involved in hybridization
rZoJ’SZ species; they may be two strains, varieties or
races of the same species. It is also known as intraspecific kybridiza-
/' °thJ n Cr <° P im P rovemen£ programmes, intervarietal hybridization
^^^ommoniyused. Infact.it if so common that it may
often appear to oe the only form of L;-> riiizasion used in cron
SSSSSS. Kvo^f pie or ,£
Hybridization : Techniques and Consequences
141
i Simole Cross. In a simple cross, two parents are crossed to
IrJZfc f, The is selfed to pro^e ft o, „ used ,n .
backcross programme, e.g., A X B F % (A x IS)
o Comdex Cross. More than two parents are crossed to produce
?i n»hr?d which is then used to produce or i$ used m a backcross.
Such's cross is also known as convergent cross because this crossing
nromrame aims at converging, i.e., bringing together genes from
severalTrents into a single hybrid. A few examples of convergent
several parents 1 Fj | 7 L As crop improvement progresses,
the crop varieties would accumulate more and more favourable
lenes This would lead to greater similarities between even unrela-
ted varieties. In view of this, it may be expected that in future
mmn'ex crosses would become more and more important. In breed-
ing of highly improved self-pollinated crops like wheat and rice,
complex crosses are a common practice today. Complex crosses
would become routine in near future in the improvement of other
X pollinated crops with the progress in the level of them improve-
men!.
f>ktant Hybridization. Distant hybridization includes crosses between
different species of the same genus or of different genera. When
two snecies of the same genus are crossed, it is known as interspecific
hybridization ; but when they belong to two different genera, it is
termed as intergeneric hybridization. Generally, the objective of
such crosses is to transfer one or few simply inherited characters
like disease resistance to a crop species. Sometimes, interspecific
hybridization mav, be used for developing a new variety, e.g.,
Clinton oat variety was developed from across between A vena
sativa x A . byzkntina (both hexaploid oat species), and CO 31 rice
variety was developed from the cross Oryza sativa var. indica X O.
nerennis Almost all the present-day sugarcance varieties have been
developed frofa, complex crosses between Saccharum officinarum
(noble canes), S. barberi (Indian cones) and other Saccharum species,
e a , S. spontaneum (Kans.). The improvement in fiber length of
Indlaacdtton ( Gossypium arboreum) has been brought about by cross-
ing it with American cultivated cotton (G. hirsutum). Many improved
varieties have resulted from such crosses. Intergeneric hybridization
may also be used to develop a new crop species, e.g., Triticale from
a cross between Triticum sp. and Secaie cereale (rye). Wild species
often provide genes which are not present in t-he cultivated species.
For example, many of tre genes for rust resistance in wheat ,«re
derived from related wi ! species. Distant hybridization is likely
to become increasingly ii .ortant in the correction of specific defects
of crop species. In many tses, wild species may contribute valuable
‘yield genes’ as well to tk cultivated species.
THE HYBRIDIZATION PROGRAMME
Hybridization is the most important method of crop improve-
ment. The process of hybridization itself is fairly simple and easy.
14 :
Plant -Breeding ; Principles and Methods
Three Parents (A, B, C)
AxB
Fi (AxB) X C
Complex (A X B) X C
hybrid
Four Parents (A, B, C, D)
AXB C x D
Fi (A X B) x (C X D)
Complex (A X B) X (C X D)
hybrid
Eight Parents (A, B, C, D, E, F, G, H)
A X B C X D E x F G x H
I I I
I ! i
1 1 |
Fx (A X B) x (C x D) (E X F) X (G X H)
'l'
[(A X B) X (C X D)] x [(E X F) x (G X H)]
4 -
Complex [(A X B) X (C X D)] X [(E X F) X (G X H)]
hybrid
Fig. 7.1. Complex crosses involving 3, 4 and 8 parents.
143
Hybridization : Techniques and Consequences
Consequently, a beginner may be tempted to make many crosses.
But the difficulty in using hybridization in crop improvement lies in
the handling of segregating generations. The problems that a
breeder has to face are given below.
Raising the Segregating Generations. Generally, F a and later genera-
tions consist of several thousand plants. Raising of F 2 from several
crosses at the same time requires money, labour, land and other
• facilities. Often these facilities are but limited.
Handing of the Segregating Generations, Selection for desirable plant
types has to be made in Fa and • subsequent generations. Selection
for qualitative characters is simple .and quick, but selection for
quantitative characters is often difficult and time-consuming. If the
breeder desires to practice selection based on scientific considerations,
he will be able to handle only few crosses at a time.
These two considerations put a limit on the number of crosses
the breeder can handle at a time. Thus a breeder should not
make too many crosses only to discard most of them at a later stage
due to the lack of facilities.
Advance Planning. This problem is more important than the first
two, but is less obvious. The development of a new variety usually
takes 7-8 .-years. Therefore, a breeder has to foresee the require-
ments about 8-9 years in advance. He should be able to plan his
breeding programme according to these requirements so that he has
a veriety ready when the need arises. Thus he must know the require-
ments of the market in terms of quality etc., the diseases and pests
of the area, the environmental fluctuations and the prevalent
management practices among the farmers. This he must know for
the present time, and should also be able to expect and forecast for
the future as well. This means that the breeder must be thoroughly
familiar with all the aspects of the crop species he is working with.
Further, he must clearly define the type of variety he wants to
produce, the important characters he wishes to improve. Then
he has to search for suitable parents which will contribute these
characters, and use them in a suitable hybridization programme.
For this the breeder must be familiar with the existing varieties, and
he should be able to obtain information on the characteristics of the
germplasm collections and the wild relatives of the crop species.
In conclusion, the breeder should have well-defined, clear-cut
objectives based on the present and the expected future needs in
developing a new variety. He should select the parents accordingly
and use them in a suitable hybridization programme.
THE PROCEDURE OF HYBIDIZATION
Once the breeder has decided the objectives of programme,
he is ready to begin hybridization. There are seven steps involved
in hybridization : (1) choice of parents, (2) evaluation of parents,
144 Plant Breeding : Principles and Methods
(3) emasculation, (4) bagging, (5) tagging, (6) pollination, and
(7) harvesting and storage of F 1 seed.
Choke of Parents. The choice of parents mainly depends upon the
objective of breeding programme. In addition to other objec-
tives, increased -yields are always the objective of a breeder. There-
fore, at least one of the parents involved in a cross should be a well
adapted and proven variety in the area for- which the new variety is
being developed. ’ The other variety should be having the characters .
that are absent in this variety. In combination breeding, the genetic
diversity of the parents' is not important, but in transgressive breed-
ing genetic diversity is of great importance. For* transgressive breed-
ing, it should be made sure that the parents differ for many genes
affecting yield or some other character of Importance. However, it
would be desirable that the performance of parents is good and
that they are well adapted in the areas where they are commonly
grown. Further, some parents produce superior F k s and F& while
others do not. . This property of the parents is known as combining
ability and can be estimated by one of the many biometrical
techniques. Combining ability of the parents may serve as a useful
guide in the selection of parents for a hybridization programme.
It is essentia! that all the characters sought to be improved are
present in one or the other parent. If necessary, three or more
parents may be included in a complex cross. One should remember
that a character absent in the original parents will generally not
appear in the segregating generations, except for the possibility of
transgressive segregations and some gene interactions, e g., com-
plementary gene action. Thus the choice of parents is the basic step
in a hybridization programme and often, more than anything else,
determines its success or failure.
. Evaluation of Parents. If the performance of parents in the area
where breeding is to be done is known, evaluation is not necessary.
But if their performance in the area is not known, it should be deter-
mined, particularly for the characters they, are expected to contribute
and for disease resistance. . Disease reaction is important because an
introduced parent may be susceptible to the new races of the pathogen
occurring in the area, or even new diseases present in the area for
which their reaction may not be known. New strains should also be
checked for mechanical mixture, and for heterozygosity if the crop
species shows about 5 per cent cross-pollination. In the latter case,
it may be necessary to self-pollinate the parent for one or more
generations if it is suspected to be heterozygous.
Emasculation. The removal of stamens or anthers or the killing of
pollen grains of a flower without affecting in any way the female
reproductive organs is known as emasculation . The purpose of
emasculation is to prevent self-fertilization in the flowers of
female parent. In dioecious plants, male plants are removed, while
in monoecious species the male flowers, e.g. y in castor, or the male
inflorescence, e.g ,, in maize, are removed to prevent self-pollination.
But emasculation is essential in bisexual flowers. It may be done in
145
Hybridization j Techniques and Consequences
my one of several ways. The method suitable for a species is largely
determined by the size of its flowers, the amount of seed needed the
number of seeds per fruit and the purpose for which the hybrid seeds
are required. In species with relatively large flowers, hand emascula-
tion may be adequate in most hybridization programmes. On the
other hand, in species with small flowers hand emasculation is
generally difficult, tiring and time consuming. Similarly, when large
quantities of hybrid seed are needed, hand emasculation is impracti-
cal in most self-pollinated cr@p species, except in those that set many
seeds in one fruit, e.g., tobacco (about 2000 seeds per fruit), tomato,
brinjal etc. ' Similarly, for accurate genetic studies, hand emascula-
tion is desirable because with other methods there may be some
self-pollination.
The efficiency of an emasculation technique may be tested by
bagging the emasculated flowers without pollination. The amount of
seed thus set would indicate the frequency of chance self-fertilization
during emasculation. If the seeds are to be used in genetic studies,
there should be no self-pollination during emasculation. In most of
the crop improvement programmes and in the production of hybrid
seed, a small amount of self pollination may be permissible. But
it is desirable that self-pollination should be kept to the minimum,
preferably to the zero level. The various techniques of emascula-
tion are outlined below.
Hand Emasculation. In species with relatively large flowers, stamens
or anthers are removed with the help of forceps. The exact details of
the procedure vary from one crop species to the other. Emascula-
tion is done before the anthers are mature and the stigma has be-
come receptive to minimise accidental self-pollination. Usually,
stigma, receptivity is at its peak during the morning hours when the
flowers open, but different crop species show considerable variation
in the duration for which their stigma remains highly receptive. Thus
emasculation is generally done in the evening, between 4 and 6 P.M.,
one day before the anthers are expected to dehisce or mature and
the stigma is likely to become fully receptive. Therefore, the flowers
selected for emasculation are likely to open the next morning. With
some experience, the breeder should be able to select such flowers
without much difficulty. Generally, it is desirable to remove the
older .and the younger flowers located close to the flower to be
emasculated in order to avoid confusion in identification of crossed
pods etc.
A generalised procedure for hand emasculation, is as Follows.
The corolla of the selected flowers is opened and the anthers are
carefully removed with the help of fine-tip forceps. In many crop
species, the androecium is epipetalous,. e.g., in cotton, jute, brinjal,
sweet potato, tomato, potato, bhindi, etc. In such cases, sometimes .
the corolla may be totally removed alongwith the epipetalous stamens.
In cereals, one-third of the empty glumes may be clipped off with
scissors to expose the anthers. In wheat (T. aestivum) and oats (A.
sativa), only two large florets per spikelet are left ; the other florets
Plant Breeding :• Principles and Methods
?4fc
are removed. Care must he taken to remove all the anthers from the
flowers without breaking them and, the most important, the gvnoccium
'must not be injured . The first precaution in necessary to prevent
self-pollination, while the latter is necessary for seed set after the
desired pollination. Further, it is desirable to cause the minimum
damage to the flower during emasculation. An efficient emasculation
technique .should prevent self-pollination and produce a high percentage
of seed set on cross-pollination .
f ^ fhe tech nique of emasculation in barley (//. vulgare) is des-
cribed in some detail. A spike that is about to emerge from the flag
leaf would be generally appropriate for emasculation. The flag leaf
Is carefully peeled away, but not removed, to expose the spike. The
spikelets from the lower and the upper 2-3 nodes are clipped off On
the remaining nodes, generally the two side spikelets are removed
leaving the central spikclet only. This is done because the stigmas
of central and side spikelets become receptive at different times.
The top onedhird of ma and palea is clipped off ; the three
anthers present in each spikclet arc now easily accessible and are
removed with the help of forceps. Care must be taken not to
damage the stigma 'and ovary, and to remove all the anthers from
all the spikelets.
Suction Method . This method is useful in species with small flowers.
Emasculation is done in the morning just before or immediately after
the flowers open. The petals are generally removed with forceps
exposing the anthers and the stigma. A thin rubber or glass tube
attached to a suction hose is used to suck the anthers from the
flowers. The tube is also passed over the stigmas to stick any pollen
grains present on their surface. The suction may be produced by an
aspirator attached to a water tap, or by a small suction pump
The amount of suction used is very important. The suction should
be enough to suck the stamens and pollen grains, but not the flowers
or the gynoecium. With suction method , considerable self-pollination
(up to 15 per cent) is likely to occur. Washing the stigma with a iet
of water may help in reducing self-pollination. However, self-polli-
nation cannot be eliminated in this method.
Hot Water Emasculation. Pollen grains are more sensitive than the
female reproductive organs to both genetic and environmental fac
tors. This property is utilized to kill the pollen grains with hot-
water or other agents like alcohol treatment or cold water treatment
without damaging the female reproductive organs. In the case of
hot water emasculation, the temperature of water and the duration
of treatment vary from crop to crop, and must be determined for
every, species. For jowar (S. bicolor), treatment with water at 42 4<?°r
for ten minutes is found to be suitable. In case of rice (O <aivd\
10 minute treatment with water at 4<M4°C is adequate Hof wafer
treatment is given before' anther dehiscence and prior to openine of
flowers. The hot water is generally carried in thermos flasks and the
whole spike is immersed in the water. Emasculation with hot water
Hybridization : Techniques and Consequences 14 7
is generally highly effective in killing all the pollen grains provided
the correct temperature and treatment duration are used.
Alcohol Treatment . It is not a commonly used method of einascula-
tion. The method consists of immersing the flower or the inflore-
scence m alcohol of a suitable concentration for a brief period
followed by rinsing with, water. In sweet clover, immersion of the
inflorescence m 57 per cent alcohol for 10 seconds was highly effec-
tive ; the percentage of selling was only 0.89. It is a better method
of emasculation than the suction method. However, the duration of
treatment is of utmost importance. Even a slightly, prolonged
period of treatment, say a few seconds more than the recommended
would greatly reduce seed set. This is because the female reproduc-
tive organs would also be killed by a longer treatment.
Cold Treatment. Cold trearnent, like hot water- treatment kills
pollen grains without damaging gynoecium. In case of rice ’treat-
ment with cold water at 0-6°C kills pollen grains without affecting
gynoecium. Keeping wheat plants at 0-2°C for 15-24 hours kills the
pollen grams. Cold treatment is less effective than hot water treat-
ment. The amount of self-pollination is generally greater in cold
treatment than in the case of hot water treatment.
Genetic Emasculation. Genetic or cytoplasmic male sterilifv
may be used to eliminate the necessity of emasculation (Chapter 3 ).
Many species are self-incompatible. In such cases, emasculation is
not necessary because self-fertilization will not take place In certain
genotypes and under certain environments, the male sterility and
self-moompatioihty systems may breakdown partially. However
for commercial hybrid seed production, male sterility is the most
feasible method ot emasculation. Protogyny facilitates crossing
without emasculation. Since the stigmas become receptive before
trie anthers mature, hand pollination ensures seed set from cross-
PreV£ntS se!f - fertiIization > /■«.. in bajra (/>. ameri-
Bigging. Immediately after emasculation, the flowers or the infiore
scences are enclosed in suitable bags of appropriate size to prevent
random cross-pollination. In cross-pollinated crops, like maiVe the
male flowers are also, bagged to maintain the puritv-of pollen used
for pollination. The bags may be made of paper, butter paper
glassme or fine cloth. Butter paper or vegetable parchment b nos
are the most commonly used. Cloth bags a°re genemfly not prf
forred since they permit some degree of chance cross-^lUrL?
The bags are tied to the base of inflorescence or to the stal'-' of
flower with the help of thread, wire or pins designed for the nuf
pose. The moisture and temperature are .generally hiehef insfd^
bags as compared to the outside. Therefore baeeiiMrm-fv « f ^
fungus development on the fruit or the snike’ Thif S ^f= S P ronio{e
by removing P the bags after
usually 2-3 days after pollination. ^^-polhnation is over
Tagging. The emasculated flowers are tavo-pd h, ct v.
Tag S are ...liable i. different site,. £ crops, dffS
148
plant Breeding : Principles and Methods
tags of about 3 cm diameter, or rectangular tags of 3x2 cm are used.
In crops like maize (X. niavs) y bajra (P . americanum) and jowar
(S. hicolor), bigger tags (6x 3 cm) are used. The tags arc attached
to the flower or the inflorescence, with the help of thread. J he
following information is recorded on the tags with a carbon pencil.
L Date of emasculation
2. Date of pollination
3 , Names of the female and the male parents. The name of the
female parent is written first, and that of the male parent, is
written later, e.g. 9 A >< B denotes that A is the female parent and
B is the male parent.
Pollination. The two most important operations that determine the
amount of seed set in hybridization are emasculation and pollina-
tion. During emasculation,- damage to the female reproductive %
organs must be avoided. hi cose of pollination , -mature* fertile and ^
viable pollen should be placed on a receptive stigma to bring . about
fertilization. The duration of pollen viability after anther dehiscence
varies greatly from one species to another, c.g.> a few minutes in
wheat and oats to a few hours in maize. Therefore, it is advisable
that fresh pollen from mature anthers should be used for pollination.
The time of anther dehiscence falls within the duration of stigma
receptivity and both generally coincide with^ the opening . of
flowers. Anthers generally dehisce during morning ; the exact time
varies with the species.
The pollination procedure consists of collecting pollen from
freshly dehisced anthers of the male parent and dusting this pollen on
‘'the stigma of emasculated flowers. This may be done in one of the
following several ways. J
(i) Pollen grains are collected in a bag, and are used for dusting
stigmata of female inflorescence or of emasculated flowers,
the e,g in maize, bajra etc.
(ii) Mature anthers are collected from the flowers of male parent.
The pollen is liberated and applied, to the stigma with the help
of a camel hair brush, pieces of paper, tooth pick or forceps.
(iii) Anthers are collected and allowed to burst directly over the
stigma. In rice, oats, wheat and barley, one anther is generally
inserted in each floret where it dehisces and covers the stigma
with pollen grains.
(iv) The spike of male inflorescence is shaken over the emascu-
lated inflorescence just when the anthers are about to dehisce.
As a-»result, the exposed stigma are covered with pollen. This is
commonly done in wheat and barley where the lemma and palea
are clipped off to expose the stigma of emasculated flowers.
The lemma and palea of the spike of male parent are also
clipped of to expose the anthers, which are used as the source
of pollen.
(v) In species like maize, the male inflorscence may be detached and
enclosed in the bag covering the female inflorescence. In case of
Hybridization : Techniques and Consequences iw
bajra and jo war, panicles from the male parent may be enclosed
in the same bags that enclose the panicles of female parent.
Harvesting and Storing the F, Seeds. The crossed heads or pods
should be harvested and threshed. The seeds should be dried and
properly stored to protect them from storage pests. Proper care
should ’be taken to avoid contamination of the hybrid seed with
other seeds. The seeds from each cross should be kept separately
and, preferably, the seeds should be kept alongwith the original
tags.
RAISING THE F x GENERATION
If the parents involved in a cross differ for genes affecting
seedling characteristics, the parent with the dominant character
should be used as the male parent. This would allow the identifica-
tion of selfed seeds in the F s generation. The breeder should take
advantage of such marker genes if they are available in the cross he
has planned.
There is some controversy regarding the size of F t . Allard feels
that even 12 F x seeds are enough for most breeding progammes,
except the backcross. But Elliot is of the opinion that the Fx popu-
lation should be as large as possible, because Fx is the only genera-
tion where all the genes for which the two parents differ would be.
in the heterozygous condition. Thus the larger the size of Fj s the
greater the opportunity of rare recombinations to occur. Generally,
a larger F% size is not used because (1) hand crossing is tedious and
time taking, (2) with a large F\ population, the F 2 becomes un-
manageable, (3) many breeders feel that the size of Fx is not impor-
tant, and (4) rare recombinations may not be detected due to the
masking influence of environment. But wherever possible, a
larger F } population is more desirable than a smaller one. The F* is
usually allowed to self-poilinate, but in a backcross programme it- is
crossed to one of the parents,
SELFING
The objective of selfing is to avoid cross-pollination and to
ensure self-pollination , The technique of selfing varies from one
crop to the other depending upon the mode of reproduction. In self-
pollinated crops, selfing is the natural mode of reproduction, and to
ensure selfing no operation is needed. But in the case of often cross-
pollinated species, the flowers are generally bagged to prevent cross-
pollination. In case of cross 'pollinated species with bisexual flowers
or with both male and female flowers in a single inflorescence, bagg-
ing the entire inflorescence, or sometimes the whole plant is adequate.
The. bags may be shaken daily to help pollen dissemination and
pollination. In legumes like alfalfa (M. saliva), hand tripping of
flower is essential for self-fertilization since the stigma has a waxy
covering which must be removed to make it receptive. In a crop like .
maize, the male and female inflorescences are bagged ; the pollen is
collected in the, tassel bag and dusted on the silk of the female
150
Plant Breeding : Principles and Methods
inflorescence. Alternatively, the tassel may be cut and enclosed
in the bag covering cob. The cut end of tassel may be kept in
water contained in a small bottle to keep the tassel alive for a longer
period.
DIFFICULTIES IN HYBRIDIZATION
Ordinarily intervarietal hybridization presents little problem,
if any, while distant hybridization is often beset by many difficulties.
In many crops, however, some varieties, particularly when used as
the female parent, show a considerably lower seed set than other
varieties. This problem is not a serious one, and can be easily
resolved by pollinating a proportionately larger number of flower
buds.
A more serious problem of hybrid (F x ) mortality due to lethal
genes is encountered" in several cross combinations of some crops.
An excellent example is furnished by wheat {Triticum oestimm)
where F x plants from many cross combinations show hybrid
necrosis. Leaves of necrotic plants show discolouration and drying
of leaves which begins at the tips of older leaves and progresses
towards their base (leaf sheath). The affected areas of leaves first
turn darker green, then greyish brown as the chlorophyll degene-
rates and finally the leaf tissues die. The hybrid necrosis in wheat
is believed to be the result of two dominant complementary genes
Ne i and Ne t located on the chromosomes SBL and 2 B s } respec-
tively. According to an estimate, about 18% of wheat varieties
possess Ned while more then 42% of the varieties have iVc 2 .
There is a considerable variation in the severity of hybrid
necrosis in different cross combinations of wheat. In severe cases,
necrosis may develop in the first or second leaf stage and the plants
may die by the 3-6 leaf stage. In mild cases of necrosis, only leaf
tips may be affected and the plants may grow and reproduce
normally. All the grades between these two extremes are known
to occur and a 0-8 scale has often been used for scoring the inten-
sity of necrosis. This variation in the intensity of necrosis is
presumably due to multiple alleles of both Ne x (Ned 0 , Nei m and Ne x s
and Ne 2 (Ne 2 w , Ne 2 m 9 Ne 2 ms and Ne 2 & ) genes which have weak (w),
moderate (m) 9 moderately strong (ms)' or strong (s) necrotic effects.
The problem of hybrid necrosis (or other lethal genes) may
be overcome by (1) keeping necrotic crosses out of the breeding
programme ; (ii) using one parent having recessive alleles of both
the Ne genes ; (iii) inducing mutation for the recessive allele of
the concerned Ne gene ; (iv) utilizing spontaneous mutants having
the • recessive allele of the concerned Ne gene, where available ;
(v) using a chemical treatment, e.g., spray of an amino acid, such
as, proline, in the case of wheat and (vi) growing the plants
in an environment, if known, which will prevent the expression of
the lethal gene(s), e.g., temperatures over 22°C in the case of Ne
genes of wheat.
Hybridization : Techniques and Consequences
CONSEQUENCES OF HYBRIDIZATION
In self-pollinated species, it is the easiest to permit self-polli-
nation in fi to produce an F* generation. Segregation and recom-
bination of genes would produce several new genotypes, in addition
to the two parental types, in F&. The number of genotypes pro-
duced in jp 2 increases rapidly as the number of segregating genes
increases (Table 6.1). The number of phenotypes with and without
dominance also increases rapidly with an increase in the number of
segregating genes. If, for example, 10 genes are segregating, the F 1
would produce 1,024 types of gametes, 59,049 dilferent genotypes in
F. \ i9 and 1,024 phenotypes with full dominance and 59,049 phenotypes
without full dominance and epistasis. The smallest size of a perfect
F a population (that includes at least one plant of every genotype)
would be 1,084,576. Most of the intervarietal crosses would differ
for many more genes. It is, therefore, impractical to raise a perfect
f 2 population for any cross. Thus the difficulty in recovery of rare
recombinants from segregating generations can be. easily appreciated.
This difficulty would be 'great enough even if the phenotypic effects
of the genes were easily recognisable. However, in the case of quanti-
tative characters the effects of genes are masked' by those of the
environment and gene interactions making the identification of rare
recombinants even more difficult.
In F 2 , a vast majority of plants would be heterozygous for one
or more genes. Heterozygosity reduces the effectiveness of
selection because such plants segregate to produce variable progeny,
the genetic value of progeny is not the same as that of the'
parent plants. The frequency of completely homozygous plants in
any segregating generation is given by the following formula.
Proportion of completely homozygous plants ==[(2™ — 1)J 2 m ) n
where m is the number of generations of self-pollination and n is the
number of genes segregating. As the number of generations of - self-
pollination increases, the frequency of completely homozygous plants
increases sharply (Table 7.2). Consequently, the confusing effects
of heterozygosity are reduced and selection becomes more effective
in the advanced selfed generations. Many breeders feel that selec-
tion on individual plant basis should be delayed till F 5 or Fo genera-
tion.
These considerations have assumed equal survival of all the
genotypes, and no linkage. There is evidence that heterozygotes
may he favoured in nature. The increase in homozygosity, therefore,
would, be less than theoretically expected. Linkage would decrease
the frequency of recombinant types and increase the frequency of
completely homozygous plants. The latter effect is produced because
linkage reduces the number of genes segregating independently.
Linkage between favourable genes, makes selection easier because
the two desirable characters tend to be inherited together. ■ But
linkage between favourable and unfavourable genes is a problem
because larger populations would be required to break this linkage.
Plant Breeding Principles and Metnoas
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Hybridization : Techniques and Consequences 153
Tat)Ie 7.2. Frequency of completely homozygous'' plants in Fa and the
subsequent generations of crosses segregating for d liferent number
of genes.
Number of genes Frequency (in per cent) of comp l etely homo zygous plants
segregating
Ft
Fa
f 4
h
F 7
1
50.0
75.0
87.5
93.8
98.4
2
25.0
56.3
76.6
87.9
96.9
3
12.5
42.2
67.0
82.4
95.4
4
6.3
3L7
58.6
77.2
93.9
5
3.1
23.7
51.3
72.4
92.4
10
0.097
5.6
26.3
52.5
85,4
20
0.00009'
0.32
6.9 ■
27.5
' 73.0
* The number of generations of self-pollination will be 1 in FT 2 in F ; », ,3 in
F 4 and «-i in Fn generation since FT is the result of hybridization and
only Fa and the subsequent generations ate due to selling.
Iii case of quantitative characters, recombination may produce
genotypes superior to the two parents. For example, if a genotype
AA bb CC dd is crossed with another variety with the genotype
aa BB cc DD , the F 1 would be A a Bb Cc Dd , and in F% plants with
genotypes A A BB CC DD and aa bb cc dd would be produced. These
plants would be superior and inferior, respectively, to both the
parents. Such recombinants are known as transgressive segregants.
Breeding far yield generally aims at the recovery of transgressive
segregants. But if one considers the frequency of such segregants in
Fa, which is 2 out of 4 n plants (where n is the number of genes
segregating), it would be apparent that the chances of recovery of
such segregants are indeed very lew. Most of the plants would,
however, have some good genes from each parent, but rarely a plant
in a small F z population would have all the good genes from both
the parents.
SUMMARY
Hybridization consists of crossing of two plants or. lines with different
■ genotypes. The purpose of hybridization is to. bring together in Fi genes from
the two parents for the purpose of combination or transgressive' breeding , The
Fi may be used as a variety where it is commercially feasible. The two parents
involved in a cross may belong to the same variety, different varieties of the
same species (intervqnetal a r intraspecific hybridization . ), or to different .species
of the same genus or to different genera (distant hybridization ), The inter**
varietal hybridization may involve. two ( simple cross) or more parents (complex
cross). The function of distant hybridization generally is to transfer specific
characters to a cultivated species*
The breeder should have clear-cut objectives in making a cross, ana the '
parents should be selected to fulfil these objectives,. The parents are evaluated'
tor various characteristics, before being, crossed. The flowers ;\bf ' parent: to
be used as female are emasculated by hand, suction, hot, cold or alcohol treat*'
pent, male sterility or self-incompatibility. The emasculated- flowers are
immediately bagged and tagged. Emasculation Js done one -day before the.
Plant Breeding : Principles and Methods
receptive,^ usually in the evening between 4-6 P.M. The
« are Pollinated by hand the next morning. It is 'desirable
as the resources P ermit provide tbs "maxi-
*7 segregation and recombination produce a large
with number of different genotypes possible in P
w,t h an increase in the number of segregating genes
rap.-diy with continued seifing. The frequency 8 ®}' comr
a so increases rapidly. By F„ about 73 f
er ol genotypes in
tses geometrically
zygosity increases
r lomozygous plants
per cent of the plants become com-
jenes are -segregating. Transgressive segregation
•every of such recombinants will be verv difficult
QUESTIONS
What is hyridizaiion ? Briefly describe the difte
Discuss m detail the objectives of hybridization
Describe in brief the various steps involved
Comment on the following : (i) Parent;
diverse, <ii) Linkage increases the i
plants as well as homozygosity, (iii) fra-
breeding, and (iv) Perfect population
oinauons from an inteivarietal hybrid.
Define emasculation. Briefly describe the various
Write short notes on the following, (i) Pollination
i'® n ’ Bagging, (iv) Hot water emasculation i
tor d i hybridization programme, (vi) Complex era
breeding, and (viii) Transgressive breed ! ns
:n. hybridization.
Es- involved in hybridization should be
frequency of completely homozygous
ransgressive segregation'" in plant
for the recovery of all. gene' com-
‘ Genetic eoiascula-
'election of parents
(vii) Combination
Chapter 8
Genetic Composition of
Cross-Pollinated Populations
Cross-pollinated crops are highly heterozygous due to free
ioterniating among their plants. They are often referred to as random
mating populations because each individual of the population has
equal opportunity of mating with any other individual of that
population. Such a population is also known as Mendelian popula-
tion or panmictic population. A Mendelian population may be
thought of having a gene pool consisting of all the gametes produced
by* the population. Thus the gene pool may be defined as the sum
total of all the genes present in a population. Each generation of a
Mendelian population may be considered to arise from a random
■ sample of gametes from the gene pool of previous generation.
For this reason, it is not possible to. follow the inheritance of a gene
in a Mendelian population by using the techniques of classical
genetics. To understand the gcnttic make-up of such populations a
sophisticated field of study, population genetics , has been developed.
We shall examine the elementary principles of population genetics
In order to understand the genetic composition of random mating
populations.
THE HARDY- WEINBERG LAW
The Hardy- Weinberg law Is the fundamental law of population
genetics and provides the basis for studying Mendelian populations.
This law was independently developed by Hardy (1908) in England
and Weinberg (1909) in Germany.- The Hardy -Weinberg law states
that the gene and genotype frequencies in a Mendelian population
to remain constant generation after generation if there is no selection ,
mutation , migration or random drift . The frequencies of the three
genotypes for a locos with two alleles, say A and a , would be
p 2 AA 9 2 pq Aa> and q 1 aa ; p represents the frequency of A and q
represents the frequency of a allele in the population. The sum of p
and q is one, i.e. 9 p+q= 1 . Such a population would be at equi-
librium since the genotypic frequencies would be stable, that is,
156
Plant Breeding : Principles and Methods
would not change, from one generation to the next. This equilibrium
is known as Hard}- Weinberg equilibrium , A population is said to be
at equilibrium when frequencies of the three genotypes, AA , Aa
and aa are p\ 2 pq and q 2 , respectively. Whether a population is at
equilibrium or not can be easily determined by a chi-square
test.
Hardly-Weinberg law can be easily explained with the help of
an example. Let us consider a single gene with two alleles, A and a,
in a random mating population. There would ^ be three ^ genotypes
AA, Aa and aa. Suppose the population has .V individuals of which
D individuals are AA, H individuals are Aa and R individuals are
aa so that D + H+R— Ah The total number of alleles at this locus in
the population would be 2N since each individual has two alleles at a
single locus. The tola! number of A alleles would be 2 />-{*// because
AA individuals have two A alleles each, while each Aa individual has
only one A allele. The ratio (2D 4* H)/2N is, therefore, the frequency
of A aUefe in the population, and is represented hyp. Similarly,
the ratio (2R4~H)f2N is the frequency of a allele, and is written as
q. Therefore,
p=(2D \ H)I2N or
and
q=(2R+H)i2N or
«C R+iH)/N
Therefore, p+q=\ or
p—l-Qt or q~\~p
The values of p and q are known as gene frequencies. Gene frequency
is the proportion of an allele, A or a, in the population. In other
words, the proportion of gametes carrying an allele, A or a, is
known as gene frequency. The genotype frequency or zygotic fre-
quency is the proportion of a genotype, A A. Aa or aa , in the popula-
tion. Random mating or random union of the two types of
gametes would produce the following genotypes in a ratio
proportionate to the frequencies of gamates uniting to produce
the zygote.
Thus the genotypic frequencies are : p 2 AA, 2 pq Aa and q 2 aa .
According to the. Hardy- Weinberg law, the frequencies of
two types of gametes produced by this population would be p'(for A)
and q (for a). Therefore, the frequencies of genotypes in the
next generation will be the same as those in the present generation
Genetic Composition of Cross- Pollinated Populations 357
that is P AA ' 2 PQ Aa and *? a aa - This cai1 be easily shown as
follows. ‘ In the present generation,
AA Aa
Genotype
Genotypic frequency p* 2 pq <7*
thic population would produce two types of garnets A and a. The
‘Yeouencies of gametes containing A and a ailclcs can be ca.cmat-
s'd in a similar manner as described before. It may be noted that
^-p*, //.-> 2pq and R=q 2 . Further, N= 1 since,
p iJ r2p(i-\rq 2 =(p+qY and
p-f-q— 1, hence
p- -\-2pq 1
The frequencies of A and a gametes may be calculated as
•••*, follows.
• ? Frequency of gametes containing A allele
^ ' —(D + hH)IN
—(p n +pq)l 3
=• p*+pq
Similarly, the frequency of gametes containing a allele
==’( R+lH)!N
==(< ? A .-pq)H
=q*+pq
It can be easily shown that p % -\-pq—p-
p-A-pq^p ( p+q )
=/? (since p-f-q— 1)
Similarly, q*A-pq= q (q+p)
* —q (since q-\ p—l)
It is clear that the frequency of the two types of gametes is the same
as in the previous generation. It can be similarly shown that the
frequencies of the Three genotypes in this generation would also be
the same as those in the previous generation. Random union of the
male'and female gametes would produce the following genotypes.
p 2 AA
pq Aa
pq Aa
q z aa
Thus the genotypic frequencies would be p- AA, 2 pq Aa &nd q~ aa,
which is the same as in the previous generation. It can he shown
that r aMom mating between various genotypes would lead to the
same result. The frequency of mating between identical genotypes,
q a between AA and AA, Aa and Aci or act and aa would equal
tntb- smiare of the frequency of that genotype. For example, the
frequency of mating Aa x Aa would be (2 pq)\ U 4 p q*. But the
frequencies of matings between two different genotypes, e.g. t AA and
158
Plant Breeding : Principles and Methods
Aa, AA and aa etc., would be twice the product of the frequencies
of the two genotypes. This is because in such cases two combina-
tions are possible. For example, the mating between AA and aa
could be either as AA as a male (aaxAA) or aa as male (AAxaa).
The frequency of this mating, therefore, would be 2 (p 2 xq 2 ) or
2p i q 2 . Table 8.1 lists the frequencies of various types of matings
and the progeny produced from them.
Table 8.1.
Consequences of random
population,
mating
of genotypes in a
Mendelian
Mating
Frequency of mating
Frequency of progeny from the matin#
AA
Aa
aa
AAXAA
p'xp*=p*
P*
A A x-Aa
2(p 2 X 2pq) =•= 4p s q
V?
v<?
AAxaa
2(p*Xq*)^2pY
2p-q %
AaXAa
(2 pqX 7 pq)z=4p 2 q*
p*r
2pV
p~q 2
Aa X aa
2(2 pqXq 2 )
w
Ipq*
aaxaa
q *Xq 2 ^q*
The frequency of progeny from a mating is deduced by a simple
logic. For example, mating AAXAA would produce only one type
of progeny, A A. The frequency of progeny from a mating would
be proportionate the frequency of the mating, e.g., p\ in the
above case. The matting A A X A a would produce two types of
progency AA and Aa in equal proportions. The frequency of the
mating is 4p 3 q. Therefore, the frequency of progeny would be
2p 3 cj AA and 2p 3 q Aa. Similarly, the frequencies of different
progenies produced by other matings could be worked out.
The frequency of progeny with AA genotype would be,
—P i +2p 3 cj+p 2 q 2
=p 2 (p t +2pq+q°)
~P 8 (since p*+2pq+ q 2 = 1)
Similarly, the frequency of aa progeny would be,
=p 2 q 2 A-2pq z +q i
—<■. f(p-+2pq+q”)
=tf 2 (since /A-f 2/?? + ;/-== I)
And the frequency of Aa progeny would be,
=2pV+2pV+2/>V+2M 3
=2p i q-\-Ap 2 q 2 A-2pq' i
=2 pq (p 2 +2pq + q 2 )
—2pq (since p 2 +2pq+q-=\)
Thus it can be seen that random mating of genotypes also produces
the three genotypes AA, Aa and aa in the frequencies p~, 2 pq and a\
Genetic Composition of Cross-Pollinated Populations
159
respectively. Thus it is clear that in random mating populations the
gene and genotype frequencies remain constant generation after
generation. '
When genotype frequencies are disturbed and the population is
not at equilibrium, -it reaches equilibrium after one generation of
random mating. But when we consider two genes together, the
equilibrium is not restored in one generation, but the approach to
equilibrium is very rapid. Linkage reduces the rate by which a
population . reaches equilibrium. However, when many genes are
considered together, the approach to equilibrium is very slow. It may
be pointed out that strict random mating seldom, if ever, occurs in
cross-pollinated crop species, particularly in the small populations
that breeders maintain.- Furthermore, the genotype and gene
frequencies are affected by selection, mutation and migration. We
shall briefly consider the effects of these forces on gene and geno-
type frequencies in Mendelian populations.
FACTORS DISTURBING THE EQUILIBRIUM IN
POPULATIONS
The equilibrium in radom mating populations ’is distubred by
migration, mutation, selection and random drift ; these factors are
also referred to as evolutionary forces.
Migration
Migration is the movement of individuals into a population from
a different population . Migration may introduce new alleles into the
population or may change the frequencies of existing alleles. The
amount of change in -gene frequency q will primarily depend upon
two factors : first, the ratio of migrant individuals to those of the
original population and second, (he magnitude of difference between
the values of q in the population and in the migrant's. In plant
breeding programmes, migration is represented ~ by intervarietal
crosses, polycrosses etc., wherein the breeder brings together into a
single population two or more separate populations.
Mutation
Mutation is a sudden and heritable change in an organism and
is generally, due to a structural change in a gene. It is the ultimate
source of all the variation present in the biological materials.
Mutation may produce a new allele not present in the population or
may change the frequencies of existing alleles . However, since the
mutation rate is generally very low, i.e approximately IGF 6 , the
effects of mutation on gene frequency would be detectable only after
a large number of generations. Therefore, in breeding populations
such effects may be ignored. A desirable mutation may prove very
useful when it is discovered. But a systematic use of mutuations in
crop improvement would not be feasible until techniques for directed
mutagenesis have been perfected. Directed mutagenesis implies that
the experimenter should he able to induce a high frequency of the
desired mutations through-certain techniques. At present, directed
mutagenesis is an ideal which is yet to be achieved even partially.
160
Plant Breeding : Principles and Methods
Random Drift
Random drift or genetic drift is a random charge in gene
frequency due to sampling error. Random drift .occurs in small
populations because sampling error is greater in a ' smaller popu-
lation than in a large one. Ultimately, the frequency of one
of the alleles becomes zero and that of the other allele becomes one*
The allele with the frequency of one is said to bz fixed in the popu-
lation because there would be no further change in its frequency. It
may be expected than in a small population all the genes would
become homozygous, or would be fixed in due course of time.
. Breeding .populations arc generally small, hence a certain amount of
genetic drift always occurs in them. The breeder cannot do anything
to prevent this genetic drift, except to u sc very large populations
which is often not practicable. Alternatively^ he may resort to pheno-
typic disassorlative mating which would again require time,' labour
and money.
Inbreeding
Mating- between individuals sharing a common parent in their
ancestry is known as inbreeding . In small populations, a certain
amount of inbreeding is bound to occur. Inbreeding reduces the
proportion of heterozygotes or heterozygosity and increases the
frequency of hornozygotes or homozygosity. The rate of decrease in
heterozygosity is equal to IjlN (AT* 3 number of plants) per genera-
tion in monoecious or hermaphrodite species. In dioecious species
and in monoecious species where self-pollination is prevented, the
decrease in heterozygosity is somewhat lower ; It is equal, to 1/(2 A r 4 1)
per generation. Thus in small populations, even with strict random
mating or even with outcrossing -the frequency of hornozygotes
increases, while that of heterozygotes decrease.^ due to inbreeding.
Selection
Differential reproduction rates of various genotypes is known as
selection . In crop improvement, selection is very important because
it allows the selected genotypes to reproduce, while the undesirable
genotypes are eliminated.* Thus the breeder is able to improve the
various characteristics by selecting for the desirable types. In a
random mating population, if plants with A A or aa genotypes
are selected, the frequency of A allele in the selected population
wmuld.be 1 or 0, respectively. It is assumed in this case that A A and
aa genotypes would be identified without error. In the next gene-
ration, therefore, only A or a allele would be present, i.e, 9 the alleles
would be fixed. Here the selection against the remaining genotypes
is complete, that is, these genotypes are not allowed to reproduce.
In such cases, the disadvantage in reproduction, Le. f selection differ-
ential s, is 1 and the fitness is zero for the remaining genotypes. The
fitness of a genotype may be defined as its reproduction rate in
relation to that of other genotypes . Generally, s has values less than
Genetic Composition of Cross-Pollinated Populations
one. Further, often it is not possible to identify the genotypes with
certainty. The identification of genotypes is made difficult by
dominance and due to less than 100 per cent heritabillty. This is
particularly true for quantitative characters. As a result, selection is
expected to change gene frequencies rather than to eliminate one or
the other allele.
GENE FREQUENCY
Fig. 8.1. Change m q (Ag)per generation under constant selection intensity 00
as affected by domi nance.
When s is less than 1, the rate of change in gene frequency,
i.e., A g 9 would depend upon the intensity of selection (r) and upon
the gene frequency q. If s is fixed, that is, for a given value of y,
A q would depend upon the gene frequency q and would be
affected to some extent by the degree of dominance. The effects of
gene frequency and dominance on A# are shown in Fig. 8 1, It
can be seen that when an allele is rare in a population, ' A q due to
selection for this allele is small. As the value of q increases due to
selection, the Aq also, increases reaching a maximum at about
#=Q.3 in the case of dominant allele A, at #= 0.5 when there is no
dominance and at #=0.7 in the case of recesive allele a . After
this, A g again declines 'as the favoured allele becomes more frequent
in the population. It is evident that the effect of . dominance is . to
determine the value of q at which A q would be the maximum.
Thus selection in a random mating population is highly * effective in
increasing or decreasing the frequency of alleles ; but it is unable to
either fix or eliminate them . However, in combination with a system
of inbreeding, selection is highly efficient in the fixation and elimina-
tion of alleles.
, ■ In case selection favours the heterozygote Aa 9 both the
alleles are 'retained in the population. The q and p reach'
epuilibrium When q**sa/($a+$A), where sa and sA are the selection
O 0-2 0-4 0*6 0 ©
Plant Breeding : Principles and Methods
intensities against oa and AA, respectively. Thus the equilibrium
value of q is often different from 0.5 although at q= 0.5 the frequ-
ency of heterozygote A a, the most favoured class, is the maxi-
mum. This is because the fitness of the population as a whole is the
greatest at the equilibrium frequency of q, although the frequency of
heterozygote may not be the maximum.
Most of the characters of economic importance are quantita-
tive characters and are governed by many genes. Such, characters
show a continuous distribution. Therefore, selection intensity is
measured as the difference between mean of the population and
that of the selected individuals. This is more conveniently expressed
* a £erm . s °f standard deviation units from which the selection
differential k is calculated. The value of k is related to the per cent
of extreme individuals saved, e.g., when the extreme 5% of the
population is saved, *=2.06.
Selection of the extreme phenotype increases the frequency of
desirable alleles in the population. The /\q for each gene would'
become smaller as the number of genes governing the charac-
ter increases. With an increase in the frequency of desirable alleles,
tne trequency of desirable genotypes would also increase. New
genotypes would also appear producing more extreme phenotypes,
therefore, mean of the population would change under selection.
ISq would be the greatest when q=0. 5 since the eene action is
assumed to be additive. With a <0.5 or >0.5, the q for each gene
would become smaller. Thus selection will be unable to fix or elimi-
nate a.leles in the case of characters governed by several genes. Fiir-
foermore, variance may decrease to some extent, but selection would
show continuous gain for several generations. In case the quantita-
„,I e u, ract f IS governed by one or few genes with large effects, Aq
would be relatively large and the desirable alleles may be fixed after
^ geDer , atK)ns ° f selection - The gain under selection in such a case
k wnnlZhJ f ° r S f, Vem J Seaerations, but in the later generations
it w ould be \ery small and, more likely, discouraging,
retardIdbv P m r fo Z^J Selec£ioa . for quantitative characters is
the hmit on i1 dl 1 f g . ene . actl0n > ® iow Rentability, and (3)
ive Saract^ T SS / b 6 Se , lec£lD l n ^tensity. Heritability of quantita-
fiJthan SvV WayS l T S tha , n 00 P er cent - Therefore, there is
This M° ir ^ p0 ° dence betvveec genotype and phenotvpe.
therefore rednrns e th tlfiCatl ° n ° f 1 esirabIe genotypes difficult and,'
un ^ r selection. Permissible selection
the K d o P den£ y p ? n fec undity, i.e., reproductive capacity, of
S desires to grow 1,000 individuals
in every generation. The number of plants selected must be lamp *
enough to produce 1,000 offspring. That is, if 5 7 of foe oonulatfon
°enem1on h Fa T"!* E® 5 ° plantS (5% of f- 000 )^ Produce the next
to ohZni nnn h P ant “ r S£ produce at Jeast 20, i.e., 1,000/50, seeds
problem but fo manvnf'tZ m<?St f £be . cro P P 3ants - this is not a
prooiem, but in many of the animal species such a high reproduction
Genetic Composition of Cross-Pollinated Populations 163
rate is impossible. Another factor that limits the selection intensity
is the consideration of inbreeding. Inbreeding should be kept to a
low level to maintain heterozygosity and to avoid inbreeding
depression. As a result, the number of selected plants should be
relatively large and cannot be too small.
The above considerations are for a single character. When
two characters have to be selected for simultaneously, the selection
intensity for each of the two characters has to be relaxed. The
breeder generally has to concentrate on the improvement of more
than one character. Therefore, the intensity of seieciion for any one
character is much lower than that for a single character. For
example, if selection is practised for three characters, and 5 per cent
of the population is saved in each generation, the intensity of selec-
tion for any one character would be 3\/ 0‘05 or 37 per cent and not
5 per cent. This is because rhe selection for each character has to
be relaxed in order to accommodate selection for the other two
characters.
.SYSTEMS OF MATING
The breeder has two basic tools to change the genetic composi-
tion of populations : (I) selection, and (2) mating system. We
have discussed the effects of selection in some .detail Here we would
examine the consequences of various systems of mating. There are
five basic -mating schemes : (1) random mating, (2) genetic assorta-
tive mating, (3) genetic disassortative mating, (4) phenotypic
.assortative .mating, and (5) phenotypic disassortative mating.
Rmiom Mating
t In random mating , each female gemete is equally likely to
combine with any male gemete and the rate of reprodution of each
genotype is equals i.e. t there is no seieciion . In such a situation,
(l)gene frequencies remain constant, (2) variance for the character
. is constant, and (3) the correlation between relatives or prepotency
does not change. However, in plant breeding some form of selection
is practised ;; such a mating system is known random mating with
selection. Further, the mating is usually not completely random,
Differeacs in flowering 'time, position of plants in the field,, self-
incompatibility, and prevalent wind direction etc. limit the random-
ness of mating.
Random mating’ -with selection changes the mean of the
-character, and increases the frequency of alleles for which' selec-
tion is practised. It would tend to increase the variance when
'the selection is in favour of a rare allele, -but would tend to
reduce it when a predominant allele is selected for. These changes
are more, pronounced when the character is. highly heritable and is
•governed by one or a few genes. In case of polygenic traits with
164
Plant Breeding l Principles and Methods
1 .
2 .
3.
moderate heritability, the change due to selection is relatively smaller.
Random mating in small populations is unable to prevent an in-
crease in homozygosity due to inbreeding and genetic drift. Genetic
drift may often lead to a chance fixation on alleles.
Random mating is useful in plant breeding in several ways, e.g.,
progeny testing, production and maintenance, of synthetic and compo-
site varieties, production of polycross progeny etc.
Genetic Assortative Mating ......
In eenetic assortative mating, the mating is between individuals
that are more closely related by ancestry than in random mating
Thus in this mating system, it is not important that the genotype of
the plants selected for mating be correctly identified. This mating
system is more commonly known as inbreeding. Inbreeding has the
following effects on a population.
It increases homozygosity and reduces heterozygosity.
Alleles and, as a result, the characters become fixed, except for
the variation produed by environment. The fixation of a
character is little affected by the number of genes involved and
the heritability of the character, particularly under more
intense forms of inbreeding, e.g., self-fertilization.
Under intense inbreeding, the number of noninterbreeding
groups increases rapidly. As a result, selection must be done
to keep the population size manageable.
The total genetic variability of the population increases rapidly.
The genetic variability within interbreeding groups, i.e.,
families or lines, decreases rapidly and is ultimately reduced to
zero. ...
Under selection, genetic variability of the population decreases
rapidly. In cases where only one line is selected, the genetic
variability is very small or zero.
The prepotency of individuals increases under inbreeding.
Prepotency is the property of an individual to produce progeny
which are similar to each other and to the parent. Prepotency
is affected by homozygosity, dominance, epistasis and linkage.
Homozygosity is tho most important factor and is under
control of the breeder. As homozygosity increase the prepo-
tency of an individual also increases. An individual completely
homozygous for ail the dominant alleles will be the most
prepotent.
Genetic assortative mating is useful in the development of in-
breds, both partial and complete.
Genetic Disassortative Mating
in genetic disassortative mating, such individuals are mated
which are less closely related by ancestry than would be under random
mating. Thus in this system, totally unrelated individuals are mated.
These individuals often belong to different populations. Examples oi
6 .
Genetic Composition of Cross-Pollinated Populations 165
such a mating are intervarietal and interspecific crosses. The effects
of genetic disassortative mating are similar to those of migration.
It may be expected that this mating system would reduce homozy-
gosity and increase heterozygosity.
Phenotypic Assortative Mating
Mating betweeen individuals which are phenotypic ally more
similar than would he expected under random mating is called pheno-
typic assortative mating. Phenotypic assortative mating leads to,
1. Division of the population into two extreme phenotypes. Unlike
inbreeding which fixes intermediate types as well, under this
system of mating the intermediate types are not fixed. Only the
two extreme phenotypes, i.e. 9 the lowest and the highest, remain
in the population.
2. Increase in homozygosity.
3. Increase in genetic variability provided both the extreme pheno-
types are kept in the population.
4. An increase in the prepotency of individuals due to the increase
in homozygosity.
These effects disappear rapidly when random mating is
restored. The effects of phenotypic assortative mating become
quickly obvious in the case of thase characters that are governed by
one gene, but the effects are slower to appear in the case of those '
governed by two genes. In the case of polygenic traits, the effects
are much slower to realise, and the intermediate types are seldom
eliminated. Dominance and nonadditive gene actions further reduce
the effects of phenotypic assortative mating. Since heritability is
always less than 100 per cent for most of the quantitative traits, the
effectiveness of this mating is further reduced. Therefore, the
increase in homozygosity etc. is much slower than that expected with
100 per cent heritability.
This mating system is useful in the isolation of extreme pheno-
types. It is used in some breeding schemes , e.g., in recurrent
selection.
Phenotypic Disassortative Mating
Mating between phenotypicalfy dissimilar individuals is referred
to as phenotypic disassortative mating . The consequences of this
mating are as follows.
1* Maintenance of or even some increase in heterozygosity. ■
2. Some reduction in population variance since it tends to produce
intermediate phenotypes.
3. Reduction in correlation between relatives or in prepotency due
' to the increase in heterozygosity.
. This mating system is very useful in making a population stable ,
i.e., in maintaining variability. Suitable parents may be selected to
remove their weaknesses. The progeny from such a mating would be
more desirable than the parents, ft is also useful when the desirable
166 Plant Breeding : Principles and* Methods
type is an intermediate one and the available parents have the extreme
phenotypes . But the most notable use of this mating system is in
maintaining variability in relatively small populations as it reduces
inbreeding .
SUMMARY
In random mating populations, gene and genotype frequencies remain cons-
tant if there is no migration, mutation, genetic drift and selection. Mutation is
of little significance in plant breeding programmes. Migration is represented by
mtervarietal and interspecific crosses. It may introduce new alleles in the
population and may change gene 'and genotype frequencies. Genetic drift and
inbreeding are associated with small populations. They cause an increase in
homozygosity.
Selection is the most important too! in plant bleeding. Selection is
effective in changing gene and genotype frequencies. As a result, it alters the
mean in the direction of selection. New genotypes appear due to new gene
combinations. The variance may also be a heeled, if a character is governed
by one cr a few major genes, selection would lead to the fixation of desirable
alleles. Butin case of. ..polygenic traits, fixation does not occur. Advance
under selection is limited by dominance, nonadditive gene action and less than
100 per cent heritabiiity. Of these three factors, hentability is of the greatest
importance.
Genetic assortative mating increases homozygosity and is useful in the
development .of inbred lines. Phenotypic assortative mating eliminates the
intermediate types and divides the population into two extremes. This mating is
desirable when the extreme types are desirable. Genetic cliiassortative mating is
represented by intervarieral crosses. Phenotypic disassortative mating main-
tains heterozygosity in the population, it is a suitable mating scheme in cases
vhere the population is to be kept stable and heterozygous.
QUESTIONS
1. Explain the Hardy Weinberg law in detail with the help of suitable formulae.
2. List the factors which disturb the Hardy-Weinberg equilibrium in a Mendelian.
population and briefly describe their effects.
3. What are the different systems of mating ? Describe the genetic consequences
of these mating systems.
4. What are the differences between the following : (i) migration and introgres-
sion, (it) gene poo! and gene frequency, iiii) prepotency and fecundity, and
(iv) selection differential and selection intensity.
5. The equilibrium in a random mating population is disturbed by selection,
migration, mutation, .genetic ; drift and inbreeding. In which of the above
cases, it will be possible to return the population to the original composition
and how ? Explain with reasons.
6. Describe the following in relation to population genetics : selection diffe-
rential, selection intensity, random drift, migration, permissible selection
intensity, fecundity, prepontency, fitness of a genotype, gene frequency,
zygotic frequency.
7. In a random mating population, a character is governed by a single gene with
two alleles A and a . Assuming that the frequency of A is p and that of a is q t
compute the genotypic frequencies in the population. Snow that these frequ-
encies will remain uncharged in the next generation assuming random irnior
of gametes as well as random mating among genotypes.
Suggested Further Reading
Allard, R.W. 1960. Principles of Plant Breeding. John Wiley and Sons,
New York.
Crow, J.F. and .Kimura, M. 1970. An Introduction to Pc pulai ion Genetics-
Theory . Harper and Row, New York.
Wright, S. 1921. Systems of Mating. Genetics 6 : ill-173.
CHAPTER
Selection in Cross-Pollinated Crops
Selection in a random mating population is able to (1) change
the gene and genotype frequencies, (2) produce new genotypes'due
to the changed gene frequencies, (3) cause a shift in mean in the
direction of selection, and (4) change the variance of population to
some extent (Chapter 8). The magnitude of the.se effects is
influenced by toe number of genes controlling the character, the
degree of dominance; the nature of gene action arid, to a large
extent, heritability. The effects of these factors may be summarised
as follows.
Characters. Governed by One or Few Genes. The gain under selec-
tion would be slow if th.e. allele being selected for is rare. The gain
would become more rapid as the. frequency of selected allele
increases, ft will again become slow as the selected allele becomes
predominant in the ' population. After some, e.g ., 4-6, generations
under selection, the gain would be small; the genetic variance would
also be much smaller than that in the original population.
Polygenic Traits. When the character is governed by many genes,
the' gain under selection would be small and would continue for
many generations. The mean would change in the direction of
selection. But -the .‘.variance of the population would .not be affected
to any appreciable extent.
Dominance and nonadditive gene actions would tend to slow down
the progress under selection,. , ;
HeriiabiSity. Heritability is of great/ importance in determining the
progress under selection. Less-. than 100 per cent heritability will
cause a reduction in the progress under selection. If the heritability
is low, there may be little or no gain under selection.
A large number of studies bathe effect of selection m random
mating populations have been published. The responses to selection '
obtained in these studies can be divided into live broad groups :
(1) rapid gain, followed by slow progress, (2) continued slow
response for a long period,., (3) slow response for a short period,
168 Plant Breeding : Principles and Methods
(4) little or no respone, and (5) rapid gain —plateau -rapid gain
response.
RAPID GAIN FOLLOWED BY SLOW RESPONSE
In some cases, selection produces rapid gains for some genera-
tions. This is followed by a period of slow gain under selection
{Fig. 9,1). This type of response is typical of characters like plant
Fig. 9.1. A diagrammatic representation of rapid initial gain followed by slow
response for several generations.
height, resistance to certain diseases, days to flowering, colour etc.
Selection in the case of these characters leads to the following,
1. Change in the mean in the direction of selection.
2. Appearance of new genotypes or gene combinations due to the
changed gene frequencies.
3. Reduction in variability as indicated by variance.
Genetic Interpretation. It is suggested that such characters are
governed by few genes with major effects and several genes wi th
small effects. The period of rapid gain represents an increase in
the frequency of desirable alleles of the genes with large effects.
This increase would be large at intermediate gene frequencies; it
would become progressively smaller as the desirable alleles become
.predominant in the population. The slow gain following the rapid
gain is due to two factors. First there i% a slow increase in the '
frequency of major genes since they have become' predominant.
Selection in Cross-Pollinated Crops 169
Second, there is an increase in the frequency of minor genes, but the
rate of change in the frequency of each minor gene is bound to be
small. The slow gain, therefore, continues for a long period because
the genes would reach fixation only after a long period under,
selection.
CONTINUED SLOW PROGRESS FOR A LONG PERIOD
A suitable example of this kind of response is presented by
selection for high oil and high and low protein contents in Burr
White, an open-pollinated variety of maize (Z. Mays). We shall
consider here the findings on selection for oil content. The original
population of Burr White had a mean oil content of 4.7 per cent and
a range of 3,7 to 6.0 per cent. The mean oil content of the popula-
tion increased steadily at a low rate for 76 generations under
selection (Fig. 9.?) reaching a value of 19.0 per cent. Plants with
more than 6.0 per cent oil content were observed in the second
generation under selection. By the tenth generation under selection,
all the plants^ in the population had higher than 6 per cent oil
content. ^ This indicates the appearance of new genotypes not present
in the original population. The variability in the selected population
after 50 generations of selection was comparable to that in the
original population. The evidence for this was obtained in three
ways ; first, the variances for the different generations were compar-
able ; second , suspension of selection resulted in some decrease in oil
content, i.e ., a tendency to revert to the original Burr White ; and
third , selection for low oil content was effective. These findings
clearly indicate that a considerable variability for oil content was
present even after 50 generations of selection. Similar results were
obtained with high and low protein contents ; there was considerable
genetic variability even after 76 generations of selection. The effects
of selection may be summarised as follows.
L Slow change in mean in the direction of selection.
2. Appearance of new gene combinations after some generations
under selection.
3. Maintenance of variability even after long periods under selec-
tion.
Genetic Interpretation, Quantitative characters are generally
governed by several genes : each gene has small additive effect. In
such a case, A q. for each gene would be small. Consequently, pro-
gress under selection for such traits would be slow. The progress
would be further slowed down due to less than 1 00 per cent heritabi- •
lity. ^ As a result, genes would be seldom fixed, and there would be
considerable variability even after several generations under selec-
tion. It has been observed mat selection for one quantitative charac-
ter generally leads to changes in other unrelated characters. This is
-referred to as correlated response to selection . ^or example, selection
GENERATIONS UNDER SELECTION
Fi> 9.2,
selection in Cross-Pollinated. Crops 171
for high and low oil .and high and low protein contents in Burr
White maize brought about changes in such characters as maturity,
plant height, tiller number, cob size, grain characters and yield,
SLOW RESPONSE FOR A SHORT PERIOD
• Selection for some characters shows slow gain for several
generations which ends in a plateau. Further selection for the charac-
ter is not effective. Selection for low oil content in Burr White
maize is an .example of such a response. Oil content decreased
slowly upto 25 generations of selection ; it had decreased to about
1.0' per cent in the selected population from 4.68 per cent in. the
original Burr White. Continued selection for another 25 generations
was ineffective in further reducing the oil content below one per
cent (Fig. 9.2). The effects of selection on the population are similar'
to those in the previous case, that is, slow response for a long period,
under selection.
Most of the oil in maize seed is found in the embryo. Selection .
for low oil content was accompanied with a reduction in embryo-
size. Seed viability is likely to decrease with a decrease in embryo
size. There would be a minimum threshold embryo size below which
it will not survive. As a result, the embryo size is likely to restrict
the reduction in oil content by reducing the viability of embroys with
low oil content.
Genetic Interpretation. The oil content 'is likely to be controlled
by several genes (possibly more than 50 genes). Selection is expect-
ed to produce slow gains for several generations. The variability
will not be affected substantially by selection. That this is so was-,
evident from a comparison of the variances from selected and
unsekete J populations, trend of reversion upon suspension of selec-
tion, and the success of selection in the reverse direction. Thus there
was a considerable variability for low oil content even after ’ 25'
generations under selection. But the progress under selection was
checked due to physiological limitations, /.<?., reduced embryo-
survival due to reduced embryo size as a . consequence of the-
reduction in oil content.
Fig. 9.2, Effects of selection for high and low oil contents in the open-pollinated
maize variety Burr White. (Approximated from i.W. Dudley, i9?7*
.Proxy Int. Conf. Quantitative Genetics, eds, E. Pollack, O.Kmpthorne
and T B. Bailey. Jr,, The Iowa Stale Univ. Press. Ames, pp 459-473) The
selection for high oil content represents slow gain for many generations,
while that for low 'oil. content represents slow response for a . period
followed. by a plateau.- Continued selection for low oil produced a very
slow response after 60 generations so that after 76 generations- the mean
. for low oil line was 0.3 per cent (as compared to about i per cent after
50 -generations). - The estimated minimum number of genes- differing
between high and Jew oil populations is 54, while that between high and
low protein lines is 1221 The genetic gain udder selection was 20'xeP
for, high oil- and- high protein! (a? Is t he phenotypic standard deviation)
172 Plant Breeding : Principles and Methods
LACK OF RESPONSE TO SELECTION
Selection for some characters produces little or no gain. A
typical example of this type of response is provided by selection for
yield in maize. A large number of experiments on selection for yield
have been done. In generals, selection failed to bring about an
increase in yield. For example in one experiment the yield of
■selected population after seven generations of selection was 53.3
bushels (one bushel *=36.35 litres) per acre as compared to 53.6
bushels for the unselected variety*
Genetic Interpretation. Several experiments have shown’ that
open-pollinated varieties of maize possess considerable genetic vari-
ance for yield. , A major protion of this variance is additive,, that is,
would show response to selection. Thus the failure of yield to
respond to selection is most likely due to the low heritability of this
trait. Low heritability makes it very difficult to identify the superior
yielding genotypes. Therefore, the superiority of plants selected
.on the basis of phenotype would often be due to factors other than
their genotype. This would limit the progress under selection.
Breeding schemes utilizing replicated yield trials, eg., recurrent
selection, are capable of improving yield by selection. . This shows
that the lack of response to ' selection . for yield and some other
.characters is primarily due to their low heritability, which makes it
very difficult to identify superior genotypes on the basis of
phenotypes of individual plants.
RAPID— GAIN— PLATEAU — RAPID GAIN RESPONSE
Selection for some characters shows a period of rapid gain,
followed by a plateau, which is followed by another period of rapid
gain. Selection for increased abdominal bristle number in Drosophila
melanogaster (fruit fly) by Mather and Harrison presents a typical
example of this kind of response. The bristle number increased
rapidly ‘for 20 generations. But reproduction rate, i.e., fertility,
decreased sharply so that selection had to be stopped to maintain the
■selected population. After a few generations, fertility was restored,
but the bristle number was now much lower than that in the selected
population before selection was stopped. Selection for bristle
number in this new population was highly effective, but this time
there was no decrease in fertility. In a few generations of selection,
the bristle number increased to a new stable level. Further selection
for bristle number was ineffective, and the bristle number did not
decrease when selection was stopped. After 50 generations at this
plateau, the bristle number again showed a rapid response to selec-
tion and reached yet another plateau (Fig. 9.3). The main features*
•of this experiment were as follows.
L Rapid increase in bristle number under selection.
PLATEAU
Selection in Cross-Pollinated Crops
B38WHN 31JLSI88 1VNIWOOSV
174 Plant Breeding : Principles and Methods
2... Decrease in fertility associated with the increase in bristle
number.
3. Increase in fertility when selection was suspended, and the
accompanied decrease in bristle number.
4. Rapid gain under selection without any decrease in fertility when
selection was resumed in the new population.
5. Reaching a stable bristle number at which stage further selection
was ineffective.
6. ' After several generations, another rapid gain under selection
ending in yet another plateau.
Genetic Interpretation. It is believed that in natural populations,
the best adapted or the most lit individuals are those that are close
to the population mean for various quantitative characters. la other
words, natural selection acts against individuals with the extreme
phenotypes. Therefore, a population has to meet two opposite needs.
First, for immediate adaptation it should become phenotypically
uniform, i.e. 9 the individuals should be close to the population mean
for the quantitative characters. And second , to meet the- long term
evolutionary requirements, the population must maintain genetic
variability. It has been suggested that the population may be able
to meet these opposite requirements if only a small part of the
genetic variability is free, i.e. % is expressed, and the major portion is
hidden as potential variability. Free variability is that portion of
variability which is expressed as phenotypic difference between
homozygotes with extreme phenotypes. The free variability is,
therefore, exposed to selection. The potential variability is hidden
in heterozygotes or in homozygotes which do riot have the extreme
phenotypes and, therefore, is not exposed to selection.
Types of Potential Variability. The Potential variability is of two.
types. : heterozygotic and homozygotic.
Heterozygotic Potential Variability . This variability is stored in
heterozygotes, e g., Aa Bb etc. The heterozygotes. Aa or Aa Bb etc.
are phenotypically uniform and are very close to ' the population
mean. ^ But they would produce extreme phenotypes in the next
generation due to segregation and recombination. Tims the hetero«
zygotes function as stores of variability which is released slowly as
free variability due to segregation and recombination.
Homozygotic Potential Variability . In case of two or more genes,
homozygotes also function as stores of variability. For example,
homozvgotes AA bb and aa BB may be expected to cluster around
the mean of the population. They would, therefore, be protected
from natural selection, and would be phenotypically uniform*
However, they would produce the extreme phenotypes AA BB and
aabb after hybridization, i.e., A A bhXaaBB , followed by segregation
and recombination in the Fa. The release of homozygotic variability.
Selection In Cross-Pollinated Crops 275
therefore, requires it to be first converted into heterozygotic varia-
bility through hybridization. As a result, it is converted into free
variability more slowly than heterozygotic variability.
In the case of polygenic characters, several genes governing a
character may be present on the same chromosome.'" It will *be
advantageous to the population if these genes were linked in the
repulsion phase, i.e., some dominant genes were linked with some
recessive genes. For example, out of the four schemes for the
arrangement of. four genes; A, 3, C and D, given below scheme
number 4 would be the most desirable. This is because’ fn tS-
scheme, the full release of variability would require three crossovers
at precise points (marked X). It may be expected that namS
populations would develop such complex and elaborate sie
arrangements for storing variability. This would permit them to S
he opposing. demands of immediate fitness and long-term evoS
tionary requirements. h n CXOxU
0 )
ABC D
abed
( 2 )'
A Bed
abCD
( 3 )
a BCd
AbcD
(4)
A
b
X 3 - X
d
y ' ~D
— ■ v xy .
It is suggested that plus and minus genes affecting a ou'intita-
tive character are linked in a manner similar to that in the scheme
4 above. The genes for other quantitative characters would t
inked with these genes, and would perhaps be distributed in between
these genes. Tne initial rapid- gain under selection in e
bristle number in Drosophila ~ was most likely due to the sdecfio? ft
SStion Sel Since Q, the hlCh “ sasiiy ex .P ,ained by the above gene
wo„W bflirt d S/Sg f ™1T,
also affect the other chaeac.
response to selection may be eliminated bv crossing nv*r correlated
linked genes. When selection Ts suspended emss^o o ” 0118 1 m
have occurred to produce new g ln S ^ToSadons^ Vs a St
S5Sg“
among linked genes releasing some more of the potential v?riibffit£
SUMMARY
of seletS 0 ( n 2)" n C cre asXItXreeumcv ( if C S^ e lf he Tf n in ths direction
(3) leads to the production of new Vnodes, and^Xi^Xm^ 8inoty P- s ’
genetic variance. There are five different fvn-V n f r £ ' may ° r may not reduce
rapid gain followed by a period of riow nrinl, I™ ?° n P £ ° sei3C£ion - First,
few major genes and several minor « The Second c , h , aracter . s governed by
time is characteristic of traits ; go°em»d bv nnlt~nT' slow Wm/or a long
gene is small but the heritability is high. ThX«rrftvV- C ^> ntnbut,on of each
gain ending in a plateau. This is found in cas/of th*^ response is slow
where change due to selection beyond a lta f t charac ‘ers
limitations. The fourth type of response is XracterTJf, !fr d ^ by P^iologieal
characters that have low heritability. e.g. v Hd In snnh ° C tb ° S ? quantitative
oro duces Hh-Ih* m- nr»u_ /*. •« * **»• stich chonicters, selection
fow.
176
Plant Breeding : Principles and Methods
fieri lability. The fifth type of response, rapid gain— plateau-rapid gain,
results from the release of potential vaiiability. The initial response to selection
is due to free variability present in the form of blocks of linked genes. The
plateau presents the time during which the potential variability is slowly
released by recombination or crossing over. This release of potential
variability is followed by another rapid gain under selection. Selection for one
character is likelv to bring about change in other characters as - well. This is
known as correlated response to selection , which is due to linkage among,
genes governing different characters.
QUESTIONS
1. List the various types of responses to selection in cross-pollinated popula-
tions. Describe any two. of them and give the genetic basis.
2. Describe the rapid gain— plateau -rapid gain response to selection.
Discuss its genetic basis with special reference to the mechanism of storage
of variability.
3. Write short notes on the following : (i) correlated response to selection,
(jj) long continued response to selection, (Hi) lack of response to selection,
(iv) free variability, and (v) potential variability.
Suggested Further Reading
Allard, R.W. 1960 . Principles of Plant Breeding. John Wiley and Sons, Inc.,
New York.
Dudley, J.W. (ed.) 1974. Seventy generations of selection for oil and protein
in maize. Crop Science Society of America, Madison, Wis., U.S-.A.
Mather, K. and Harison, B.I. 1949. The manifold effects of selection. Heredity
3 : 1-52 ; 13J-162.
Woodwarth, C.M., Long, E.R. and Juqenhelmer, R.W. 1952. ‘Fifty genera-
tions of selection for protein and oil in Corn. Agron. I. 44 : 60-65.
Historical
Inbreeding depression has been recognised by man for a long.
It may not be surprising in view of the harmful effects
*11
mical bases.
INBREEDING DEPRESSION
Inbreeding is mating between individuals related by descent or
ancestry. When the individuals are closely related, in brother-
sister mating or sib ■mating, the degree of inbreeding is high. The
highest degree of inbreeding is achieved by selling. The chief effect
of inbreeding is an increase in homozygosity in the progeny, which
is proportionate to the degree of inbreeding. In fact, the measure
of degree of inbreeding is- provided by the degree of homozy-
gosity in the progeny. For example, selling reduces heterozy-
gosity by a factor off in each generation (Chapter 6). The degree
of inbreeding increases in the same proportion. In fact, the degree
of inbreeding in. . any generation is equal to the degree of homozy-
gosity in that generation.
Inbreeding depression may be defined as the reduction or loss in
vigour and fertility as a result of inbreeding .
i
178
Plant Breeding : Principles and Methods
produced by inbreeding. In many societies, marriages between
closely related individuals have been prohibited since early times.
Hindu society perhaps presents the extreme example where
marriages between individuals related by ancestry, howsoever
distant, is prohibited. But systematic observations on the effects of
inbreeding date back to about 1700 A.D. when inbreeding became a
common practice in cattle breeding. There were . considerable
improvements in production, but there .was a definite decline in
fertility. Sooner or later it became necessary to outcross the inbred
lines to increase fertility. In 1876, Darwin published his book Cross
and Self Fertilization in Vegetable Kingdom ; he concluded that
progeny' obtained from self fertilization were weaker than those
derived" from outcrossing. Darwin also reported the results from
Ms experiments on self- and cross-fertilization in maize ; these are
the first published accounts of inbreeding depression in maize.
Detailed and precise information on inbreeding in maize was pub-
lished independently by East in 1908 and by Shull in 1909. Subse-
quently, studies on other crop plants were reported. It has become
clear that in cross ; polIinated and in asexuaily propagated species,
inbreeding has harmful effects which are often severe.
Effects of Inbreeding
Inbreeding is generally accompained with a reduction in vigour
and reproductive capacity, that is, fertility. There is a general
reduction in the size of various plant parts and in yield. In many
species, harmful recessive alleles appear after selfing ; plants or lines
carrying them usually do not survive. The effects of inbreeding may
be summarised as under.
1. Appearance of Lethal mi Subletfeal Alleles. Inbreeding results in
the appearance of lethal (leading to death), sublethal and subvital
(reducing survival and reproduction rate) characteristics. Such
characteristics include : chlorophyll deficiencies, e.g., aibina, chlorina
etc., rootless • seedlings, defects in flower structure etc. Generally,
plants carrying such characteristics cannot be maintained and are
lost from the population,
2. Reduction In Vigour. There is a general reduction in the vigour
of the population. Plants become shorter and weaker because of a
.general reduction in the size of various plant parts.
3. Reduction in Reproductive Ability. The reproductive ability
of the population decreases rapidly. Many lines (plant progenies)
reproduce so poorly that they cannot be maintained. In most
of the crops, only a small number of inbred lines have enough
fertility to be easily maintained and to be useful in breeding pro-
grammes.
4. Separation of the Population into Distinct Lines. The popula-
tion rapidly separates into pheaotypically distinct lines. This is
because of an increase in homozygosity due to which there is a
Heterosis and Inbreeding Depression 1 19
random fixation of various alleles in different lines. Therefore,
the lines differ in their genotype and, consequently, in phenotype.
This leads to an Increase in the variance of the population as a
whole.
5, Increase in Homogygosffy. Each line becomes increasingly homo-
zygous following inbreeding. .Consequently, the variation within a
line decreases rapidly. Ultimately, after 7 to 8 generations of selling,
the lines become almost uniform, since they approach complete
homozygosity (> 99 per cent homozygosity). The lines which are
almost homozygous due to continued inbreeding and are maintained
through close inbreeding are known as inbred lines. Inbred lines have
to be maintained by strict inbreeding, preferably by selling, in order
to keep them homozygous.
6, Redaction In Yield. Inbreeding generally leads to a loss in yield.
The inbred lines that survive yield much less than the open-pollinat-
ed varieties from which they are derived. In maize, the. best inbred
lines yield about half as much as the open pollinated varieties from
which they were produced. In alfalfa and carrot, the reduction is
much greater, while in onions and many cucurbits the reduction in
yield is very small.
'Degrees of Inbreeding Depression
The various plant species differ considerably in their response to
inbreeding. Inbreeding depression may range from very high to
■very low or it may even be absent. The inbreeding depression ob-
served in various plant species may be grouped into four broad
categories : (I) high inbreeding depression, (2) moderate inbreeding
depression, (3) low Inbreeding depression, and (4) absence of
inbreeding depression.
High ImbrmMug Depression. Several plant species, e.g. 9 alfalfa
{M, Saliva), carrot (£>, carota ), hayfield tarweed etc., show very high
inbreeding depression.. ' A large-proportion of plants- produced by
selling show lethal characteristics and do not survive. The loss in
vigour and fertility is so great that very few lines can be maintained
.after 3 or 4 generations of inbreeding. The lines that do survive show
greatly reduced yields, generally less .than 25 per cent of the yield of
■open-pollinated varieties.
Moderate Inbreeding Depression, Many crop species, such as, maize
(Z. mays), jowar (S. bicolor ), bajra" (P. americanum) etc., show
moderate inbreeding depression. Many lethal and sublethal types
appear in the selfed progeny, but a substantial proportion of the
population can be maintained under self-pollination. There is appre-
ciable reduction in fertility and many lines reproduce so poorly that
they are lost. .However, a large number of inbred lines can be obtain-
ed which yield upto 50 per cent, of the open-pollinated varieties. '
Production and maintenance of inbred lines are relatively easier in
.these species than in those showing a high degree of Inbreeding.
180
Plant Breeding : Principles and Methods
Low fabreeiiiig Depression. Several crop plants* e.g. 9 onion ( A . cepa) 9
many cucurbits, rye (S'. cereale ), sunflower (H. annum), hemp
(C savitaX timothy grass etc., show a small degree of inbreeding
depression. Only a small proportion of the plants show^ lethal - or
subvital characteristics.. The loss in vigour and fertility is small ;
rarely a line cannot be maintained due to poor fertility. The re-
duction in yield due to inbreeding is small or absent. 'Some of the
inbred lines may yield as much as the open-pollinated varieties from
which they were developed.
No labreedlng Depression. • The self-pollinated species do not show
inbreeding depression* although they do show heterosis (Chapter-
17). It is because these species reproduce by self-fertilization and, as-
a result, have developed homozygous balance In contrast, the cross-
pollinated species exhibit heterozygous balance .
Homogzygous mi Heterozygous Balance
The concepts of homozygous and heterozygous balance were
advanced by Mather to explain the varied responses of different
. species to inbreeding. The species that reproduce by cross fertiliza-
tion are highly heterozygous. These species carry a large number of
lethal, subvital and other unfavourable recessive genes, which are
of little immediate value to the species. The sum total of these
unfavourable genes is known as genetic load . The harmful effects of
the recessive alleles are masked by their dominant alleles as a result
of which they are retained in the population. The population, there-
fore, develops a genetic organisation which favours heterozygosity.
This type of genetic organisation is known as heterozygous balance f
because it promotes heterozygosity.
' The self-fertilized species are naturally, homozygous. They have
no genetic load because unfavourable recessive genes become homo-
zygous and are eliminated from the population. These species*
therefore, develop a genetic organisation which is adapted to homo-
zygosity, i.e., which does act produce undesirable effects in the
homozygous state. This type of genetic organisation is known as
homogygous balance . The self-pollinated species have evolved from
cross-fertilized species. It is suggested that the self-fertilized species
retain sufficient heterozygous balance to show the beneficial effects of
outcrossing , i.e. 9 heterosis . The cross-fertilized species that are gene-
rally grown in very small populations, e.g., cucurbits, would show
some degree of homozygosity due to inbreeding. This would lead to-
the development of homozygous balance in such cross-fertilized
species. As a result, these species would show little or no depression
due to inbreeding.
The homozygous and heterozygous balances are concepts of
genetic organisation of populations These concepts are neither very
clear nor very specific in terms of the physical basis of genetic orga-
nisation or the types of geae combinations involved. But it may be
visualised that in self-pollinated spicies, those gene combinations
Heterosis and Inbreeding Depression 181
would be favoured which show no injurious effects in the homozy-
gous state However, upon outcrossing, these gene combinations
pay show heterosis depending upon the specific gene combinations
involved. In the cross-fertilized species, on the other hand, heterozy-
gosity is the natural condition. Therefore, gene combinations that
would be deleterious m the homozygous state are not selected
against and are maintained in the population. Consequently, such
species would show inbreeding depression. The degree of
inbreeding depression in these species would be larpelv deter-
mined by the degree of self-fertilization occurring "in the
natural populations ot the species. Cross-fertilized species like maize
mays) show upto 10 per cent • self-pollination and moderate io-
oreedmg depression. Crops like alfalfa { M . sativa) show very little
inbreeding and as a result, a very severe inbreeding depression.
I hereto re, the homozygous and heterozygous balances may be
visuaused as gene combinations that are adapted to heterozygosity
and homozygosity, respectively. uy
HETEROSIS
The term heterosis was first used by Shull in 1914 Hetn-nns
7n term f of lift ^ su P eriorit y °f an F x hybrid over both its parents
fested as in \itZ^° ther ch * racter - Generally, heterosis is mani-
mcrease in vigour, size, growth rate, yield or some other
weaker parent ThisTskn Cases h t! i s hybrid raa y be inferior to the
![. er parent This is also regarded as heterosis • we shall focus on
the S a a vSe a of tt !f at r er ' ° ften the sa P e riority of Fi is estimated over
.he average of the two parents, or the mid-parent. If the hvbrid is
rn^M 10 TV the ®. ld_I i are ^ t ’ i£ is regarded as heterosis (average hete-
retical sh^k-s^w^ 6 has f ound s ? me acceptance, particularly in theo-
flf f WeW : m practical plant breeding, superiority of the
F * ov ® r £he mid-parent is of no use since it does not offer the hvbrid
any advantage over the better parent. Therefore, averageheterosis
the_plant breeder . 'More general ly? heterosis is
s V p , enor parent >' such an estimate Is sometimes
referred to as heterobeltiosis. The term heterobeltiosk is 2 ,!!!
monly used since- most breeders regard this to be the only case of
b S ter ° s I® and re ‘ er to . lt as such. However, the commercia /usefulness
fson m d Pnmari - Iy , depend 00 its Performance ' in comp Z
nson to the be^t commercial variety of the concerned cod species Tn
many cases, the superior parent of the hybrid mav be inferior to the ’
he cosSTn rC rek[fon et io !h f aS£S ’ i£ wilJ desirable to estimate
,,„. r s s ln relarion to the best commercial variety of the cron •
mf/w es . tlI ? a£ 5 1S k pown as economic or useful heterosis Econo-
“pfaSvl’e 0 °” ,y ° f 1 4* is TcLmlrZ
used o^lvwK’n fh° W K erS K ^ Ug - ges . ted tha£ tbs term heterosis should be
naSms oTif v® h - ybnd I s euher superior or inferior to both the
dmbance Thk k?°-, S Sh °, Uid be regarded as partial or complete
minance. This is easily explained with the help of Table 10.1.
182
Plant Breeding : Principles and Method#
fable 30.1, Heterosis sod dominance in relation to parental valoes.
Position and
mke of the
Value of the
Fi hybrid
Phenomenon
>10
Heterosis
Parent A0©)
10
<10 bat >8
Complete dominance
Partial dominance
(Mid-parent)
8
,< 8 bat >6
No dominance
Partial dominance
Parent B (6)
6
Complete dominance
< 6
Heterosis
exhibits & faster development in them so that their vegetative phase
is replaced- by the. reproductive phase more quickly than m their
parents. Therefore , the me of heterosis and hybrid vigour as synonyms
Heterosis and inbreeding Depression 1 83
yielded as much as 40 per cent more than the parental varieties.
From subsequent studies on Intervarietal crosses in maize. It
became clear that some of the hybrids showed herterosis, while
others did not. Crosses between distinct types, i.e. 9 genetically
diverse varieties, exhibited greater heterosis than those involving
closely related varieties.
The commercial maize hybrids, are produced, by crossing
inbred -lines rather than open- pollinated varieties. This technique
has been successfully used in many cross-pollinated species, e.g.,
bajra, jowar etc. The development of hybrid maize is based on the
work of East and Shull beginning in the first decade of the present
century. Hybridization between inbreds developed from the same
variety or from closely related varieties produces only a small degree
of heterosis. In any case, the hybrids from such inbreds are not
superior to the open-pollinated varieties from which they were
isolated. But hybrids from unrelated inbreds generally show much
greater heterosis. Certain inbreds consistently produce better
hybrids than others. Further, certain cross combinations are-
superior to other cross combinations.
Heterosis Sss Cross- asi Seif-Pollinated Species
In general, cross-pollinated species show heterosis, particularly
when sabred lines are used as parents. In many cross-pollinated
species, heterosis has been commercially exploited, for example* in
maize, bajra, jowar, cotton (Gossypium spj 3 sunflower (H. annum),
■ onion (A. eepa), alaifa (M. sdtiva ') etc. Many: crosses in self-pollinated
species also show heterosis, but the -magnitude of heterosis^ is
generally ' smaller than that in. the case of cross-pollinated species.
But in some self- pollinated crops heterosis is large enough to be
iised for the production of hybrid varieties. Hybrid varieties are
commercially used in some .vegetables, such as tomato (L. escu-
lentum), where a single fruit produces ‘a large number of seeds.
The chief drawback m the use. of hybrid varieties in. seif pollinated
crops is mi the absence of sufficient ' heterosis , but the great difficulty
in production of large quantifies of hybrid seed .
Manifestations of Heterosis
Heterosis; is the superiority. of hybrid over its parents. This
superiority may be la- yield* quality, disease and insect resistance,
adaptability, general size or the size of specific parts* growth rate,
enzyme activity etc. These various manifestations of heterosis may
be summarised as follows.
1. Increased yield. Heterosis is generally .expressed as an increase Im
yield of hybrids. Commercially, this phenomenon is of the
greatest importance' since higher yields are the most • important
objective of plant breeding. The yield may be measured In terms .of
grain,. fruit, seed, leaf, tubers or the whole plant.
2. Increased Reproductive Ability. The hybrids exhibiting heterosis
184
Plant Breeding : Principles and Methods
show an increase in fertility or reproductive ability. This is often
expressed as higher yield of seeds or fruits or other propagules, e.g. 7
tuber in potato (5. tuberosum ), stem in sugarcane (S. officinarum)
etc.
3. Increase in size and General Vigour. The hybrids are generally
more vigorous, i.e., healthier and faster growing and larger in 'size
than their parents. The increase in size is usually a result of an
increase in the number and size of cells in’ various plant parts. For
example, fruit size in tomato, head size in cabbage (B. oleracea),
cob size in maize, head size in jowar (Fig. 30. 1.) etc.
4 Better Quality, in many cases, hybrids show improved quality.
This may or may not be accompanied by higher yields. For example,
many hybrids in onion show better keeping quality, but not yield,
than open-pollinated varieties.
5. Earlier Flowering md Maturity. In many cases, hybrids are earlier
in flowering and maturity than the -parents. This may sometimes
be associated with a lower total plant weight. But earliness is highly
desirable in many situations, particularly in vegetables. Many
tomato hybrids are earlier than their parents.
6. Greater Resistance to Diseases And Pests. Some hybrids are
known to exhibit a greater resistance to insects or diseases than their
parents.
7. Greater Adaptability. Hybrids are generally more adapted to
environmental changes than iqbreds. In .general, the variance of
hybrids is significantly smaller than that of inbreds. This 'shows
that hybrids are more adapted to environmental variations. In fact,
it is one of the physiological explanations offered, for heterosis.
8. In some cases, hybrids show a faster growth rate than their
parents. But the total plant size of the hybrids may . be comparable
to that of the parents. In such cases, a faster growth rate is not
associated with a larger size.
9. In some cases, there is an increase in the number of nodes, leaves
and other plant parts, but the total plant size may not be larger.
Such hybrids are known in beans (P. vulgaris) and some other crops.
These are some of the characteristics in which heterosis is
easily observed. Many other characters ' are also affected by
heterosis, e.g . 9 enzyme activities, ceil division, vitamin content
(vit. C content in tomato), other biochemical characteristics etc., but
. they are not so readily observable.
GENETIC BASES OF HETEROSIS AND- INBREEDING
DEPRESSION
Heterosis and inbreeding depression are closely related pheno-
mena. In fact, they may be regarded as the two opposite sides of
the same coin. The genetic theories that explain heterosis also
explain inbreeding depression. There are two main theories to
explain heterosis, and consequently, inbreedings depression : .
(1) dominance hypothesis, and (2) overdominance hypothesis.
yfV 4- ^ '&$>r ■Zf/ii5&*-i£ fiJz&afez&M
kz i *'sikxy&'f : SzS&0£
$¥ . i»>i '■ - »'§■•]' < **
leads of CS 1 1 1 (in the eeruee) and its. parents iS 8* (on the left) and
arXK 60 (on the right). Note the larger head. and gram §ize of the
hybrid as compared to its parents.
Heterosis and Inbreeding Depression
186
Plant Breeding : Principles and Methods'
Dominance Hypothesis
Tlie dominance hypothesis was first proposed by Davenport ia
1908. It was later expanded by Bruce, and by Keeble aad Feiiew in
1910. In simplest terms, this hypothesis suggests that at each, locus-
the dominant allele has favourable effect, while the recessive allele
has unfavourable effect. In heterozygous state, the deleterious effects
of the recessive alleles are masked by their dominant alleles. Thus
hetersosis results from the masking of harmful' effects of recessive
alleles bp their dominant alleles . Inbreeding depression , on the other
hand . , is produced by the harmful effects of recessive alleles, which he*
come homoyzgous due io inbreeding. Therefore, according teethe
dominance hypothesis, heterosis is mot the result of heterozygosity ;
it- is the result of the prevention of expression of harmful recessives
by their dominant alkies.. Similarly, inbreeding depression does not
result from homozygosity per se, but from the homozygosity of
recessive alleles which have harmful effects.
This hypothesis may be further explained as follows. In open*
pollinated populations, plants are highly heterozygous. As a
■ resirn, they do not show the harmful effects of the large number of
recessive alleles present in the . population. Inbreeding increases
homozygosity. As a result, ■ many recessive alleles become homozy-
gous. Lethal recessive alleles are eliminated by natural selection.
But recessive alleles with smaller' harmful effects survive la the
homozygous condition. Consequently* such alleles reduce the vigour
aad fertility of. the lines that carry them. Inbred lines are nearly
homozygous, and different inbred lines would , receive different
proportions of dominant and recessive alleles. Therefore, different
; .inbred lines may be expected to vary- in vigour and yeifd. Thus it
should be possible to isolate ihbreds. with all the .dominant alleles
present in the population. Such inbre’ds would be as vigorous: as
the open-pollinated varieties, or even more sov But such inbred's
have not been isolated so far.
Similarly, heterosis in an F\ hybrid is a result of the masking
of harmful effects of recessive. alleles present in one parent by the
dominant alleles present in the other parent and vice versa. Obvi-
ously," heterosis' would, 'depend upon the genotypes of the two
parents. Hybrids from. parents .with similar recessive and dominant
alleles would show' little or no heterosis (Fig, 10.2, cross I), while
those with different alleles would show heterosis (Fig. |0.2, cross II).
In practice, . some -parents produce heterotic progeny, while others
"do sol Generally, ' parents.. of diverse origin are more likely to
produce heterotic progeny than those of similar origin.
Objections. Two objections have been raised against the dominance
■hypothesis. The first objection relates to the failure in isolation
of lines homozygous for all the dominah'f genes. T he second objection,
is directed at the sym metrical distribution m F 2 .
1. Failure ia ffse Isolation of Inbreds. as Vigorous as Hybrids,.
According to the dominance' hypothesis, it should be possible to
Heterosis and Inbreeding Depression 187
isolate inbreds with all the dominant genes. Such Inbreds would
be as vigorous as the Fj hybrids. However, such inbreds have not
been isolated.
Cross I
Inbred A x
AA bb CC dd I
' i
Inbred B
A A bb CC dd
Fj hybrid
AA bb CC dd
NO HETEROSIS
(The same number of genes have
dominant alleles as in the two
parents. Harmful effects of b and
d genes are not -masked)
Cross II
Inbred A x Inbred D
A A bb CC dd | aa BB cc"D&
. ■ ■ i
Fj hybrid
Aa Bb Cc Dd
HETEROSIS
(Twice as many genes have domi-
nant alleles leading, to heterosis
because the harmful effects of a t
cand iiare hidden by A, B,' €
and 9)
Fig. 10.2. Heterosis as a function of genotypes of the two parents
(dominance hypothesis).
2. Symmetrical Distribution In Fg. In F 2 * dominant and recessive
characters segregate in the ratio of 3 : 1. Quantitative characters*
according* to the dominance hypothesis, therefore should not show
a symmetrical distribution in F a . This is because dominant and
recessive phenotypes would segregate in the proportion (f + i)T
'where n is the number of genes segregating. However, F a s nearly
always show a symmetrical distribution.
llxplasatloss for the Two Objections, fn 1917, Jones suggested that
since quantitative characters are governed by . many genes, they are
likely to show linkage. It may be expected that dominant and
recessive geo.es governing a character would be linked together
(Chapter 9 ; storage of variability). In such a case, inbreds contain-
ing 1 all the dominant genes cannot be Isolated because this would
require several precisely placed crossovers. It would also explain
She symmetrical curves in F gu This is often known as the Dominance
of Linked Genes Hypothesis.
Later in 1921, Collins showed that if the number
governing a quantitative character was large, symmetric
billion would be 4
188
Plant Breeding : Principles and Methods
A A and aa. Consequently , heterozygosity is essential for and is the
cause of heterosis , while homozygosity resulting from inbreeding
produces inbreeding depression . It would, therefore, be impossible
to isolate inbreds as vigorous as F x hybrids.
In 1936, East proposed that at each locus there are several
alleles, e.g., a u a% a 3 , a 4 etc., with increasingly different
functions.. Heterozygotes for more divergent alleles would be
.more heterotic than those involving less divergent ones. For
example, a x a± would be superior to a x a^ a 2 a s or It is assumed
'.that the different alleles perform somewhat different functions. ^Tfae
hybrid is, therefore, able to perform the functions of both the alleles,
which is not possible in the case of the two homozygotes.
Evidence for Overdomioa&ce. There are not rnaoy clear-cut cases
where the heterozygote is superior to the two homozygotes. This
has been the biggest objection to the- general acceptance of the
overdo mi nance hypothesis. But there Is no doubt that in the case of
some genes, heterozygotes are superior to the homozygotes. In case
-of maize (Z. mays), gene ma affects maturity. ^ The heterozygote
Ma ma is more vigorous ( and later in anthesis and maturity than the
homozygotes Ma Ma and ma ma. Gustafsson has reported two
-chlorophyll mutants in barley ( H . vulgar e) that produce larger and
more number of seeds in the heterozygous state than the normal
homozygotes. Similarly, heterozygotes for the hooded gene in.
barley show a higher rate of photosynthesis than the two homo-
zygotes.
In human beings (Homo sapiens), sickle cell anemia is produced
•by a recessive gene s 9 which is lethal in the homozygous state. In
Africa, the heterozygotes Ss are at a selective advantage over the
normal SS individuals because they are more resistant to malaria.
Another case of heterozygote advantage is reported in Neurospora
crassa (bread mould). Gene pab if concerned with the synthesis of
p-aminobenzoic acid. The heterozvgote pab* pab is more vigorous
and shows a faster growth rate than the two homozygotes pab pab
and pab*pab*.
But the number of such genes where heterozygote superiority
has been established beyond doubt is limited. There is a large
number of cases, however, where heterozygotes for chromosome
segments, e.g. 9 , inversions etc., or complex loci are known to be
superior to the homozygotes. However, the superiority of hetero-
zygotes need not be a result of overdominance. It could more easily
be due to linkages in the repulsion phase or due to epistatic effects •
(Epistasis is aa interaction between two or more nonalleles, i.e, 9
different genes. Overdominance, on the other hand, is an interaction
between two alleles.)
Comparison between Dominance and Overdomisance • Hypotheses.
The two hypotheses lead to similar expectations. But they differ
from each other with respect to one expectation. The similarities
-and differences 'between them are listed below.
Heterosis and Inbreeding Depression
189
Similarities. The two hypotheses have the following similarities.
1. Inbreeding would produce inbreeding depression.
2. Outcrossing would restore vigour and fertility.
3. The degree of heterosis would depend upon the genotypes of the
two parents. In general, the greater the genetic diversity between
the parents, the higher the heterosis.
Difference. Heterozygotes are superior to the two homozygotes
according to the overdominance hypothesis, while according to the
dominance hypothesis they are as good as the dominant honfozygote.
Therefore, inbreds as vigorous as the F t hybrid can be isolated
according to the dominance hypothesis, but it will be impossible
according to the overdominance hypothesis.
m spite of the large experimental evidence accumulated, it is
not possible to conclusively accept or reject one of the two hypo-
theses. There are definitely some genes that show heterozygote
superiority. But the number of such genes appears to be rather
small, and . even these cases couln be due to linkage in repulsion
phase or epistasis or both. It is generally accepted that heterosis, to.
a large extent, is due to dominance gene action, but epistasis and
over dominance are also involved. The relative importance of these
phenomena is not clearly understood. Recent evidence accumulated
with maize seems to suggest that overdominance may not be the
primary cause of heterosis. Overdominance is easily imitated by
epistasis and linkage, and that most reported cases of overdominance
may not represent true overdominance.
PHYSIOLOGICAL BASIS OF HETEROSIS
Earlier studies on the physiological basis of heterosis related
to embryo and seed sizes, growth, rates in the various stages of
development, rates of reproduction and of various assimilation
activities. It was suggested that hybrid vigour resulted from large
embryo and endosperm size of the hybrid “seeds as compared to
those of the inbreds. As a result, the rate' of growth in the seedline
stage may be expected to be greater in the hybrids than in the
inbreds. But these relationships were clearly demonstrated in some
cases, while in other cases they were not detectable. There is
evidence that increased size of hybrids is a result of an increase both
in the size and the number of cells. This and other observations
indicate a basic difference in the metabolic activities of hybrids and
inbreds. In 1952, Whaley concluded that the primary heterotic effect
concerns growth regulators and enzymes. He suggested that the
hybrid embryo would be able to mobilize storted food materials
earlier tfaan^ those of the inbreds * due to a more efficient enzvme
.system. This would lead to the superiority of hybrids in 'the
early seedhng stages.
l90 plant Breeding : Principles and Methods
Most of these studies concerned with the gross expressions of
heterosis, e.g., growth rate, cell number and size etc., and their con-
rSns about the physiological causes of these expressions were
Slv ?necu°Sive There was no evidence to point out clearly the
possible reasons for heterozygote advantage Recent studies demon-
strate that heterozygote superiority may be produced m several
different wavs which may differ considerably from one case to
another. These mechanisms are briefly described below.
Reduced Amount of a Single Gene Product. In some cases, the hetero-
zygote produces an intermediate amount of a gene product which
mav lead to the increased vigour and growth rate Such a situa ion
“ould be encountered when the homozygote for the dominant allele
produces more and the homozygote for the recessive allele produces
fis than the optimum level of a product. An example ot such a
situation is found in the bread mould Neurospora crassa. Gene/**
is responsible for the production of p-ammouenzcic acid. The
h'eterozvgote pab + pab produces intermediate amounts of p-ainmo-
benzoic acid and is faster in growth that the two homozygotes, that
is, pab+pab* and pab pab.
^narate Gene Products. The two alleles may produce proteins with
slightly different functions and properties. The heterozygote would
possess the oroteins produced by both the a ides, while the homozy-
Se S would have only one of the two proteins Thus the heterozy-
gous mav be expected to be more adapted to changes m the environ-
ment Consequently, they would be superior to the two homozy-
potes An example of such a situation is the sickle cell anemia in
human beings (H. sapiens). The homozygote ss does not survive
because the defective haemoglobin $ s is unable to perform the func-
tions of the normal haemoglobin $ A . The heterozygote Ss is more
resistant to malaria than the normal homozygote SS. This is because
the defective haemoglobin produced by the recessive allele s is not
suitable for the multiplication of the malaria* parasite, wmch multi-
plies in the red blood ceils. Thus 'he heterozygotes survive due to
the normal haemoglobin produced by the normal allele S. and
are resistant to the malarial parasite due to the defective haemoglo-
bin P s produced by the recessive allele.
Combined Gene Product. This is also known as the hybrid product
Manv enzymes are dimers or polymers, that is, they are made of
two or more identical polypeptides. In such cases, products of the
two alleles in the heterozygote combine to produce an enzyme mole-
cule with somewhat different properties as compared to the enzyme.s
found in the two homozygotes. This new molecule is often detectable
by electrophoresis in the heterozygote, and is often termed as the
hybrid substance. Such enzyme molecules from the heterozygotes
may often differ from the enzymes found in the homozygotes m their
physical prooerties, e.g., heat sensitivity. The two alleles governing
^he enzyme octanyl dehydrogenase-I produce heat sensitive enzymes
Heterosis and Inbreeding Depression 192
in the homozygous state. The enzyme from the fieterozygote, how-
ever, is less heat sensitive than the enzymes produced by the two
homozygotes. •
Effects in Two Different Tissues, In some cases, fieterozygote superi-
ority may arise, in a somewhat strange manner, Toe two alleles in
the homozygous state may produce high' levels of an enzyme in two
different tissues. The heterozygote may; show intermediate levels of
the enzyme in both the tissues. In maize, allele A'dh* produces more
activity of the enzyme alcohol dehydrogenase in the scutellar tissue
of seeds, while allele Adh s produces more activity in the pollen
grains. The hybrid exhibits intermediate activities in both the
tissues.
These are some of the known mechanisms that may lead tv
heterozygote superiority. There may be several yet undiscovered
mechanisms responsible for heterosis.
COMMERCIAL APPLICATIONS
Heterosis is observed in almost every crop species studied.
Often the degree of heterosis is considerably high to permit its
commercial exploitation. Heterosis is commercially used in the
form of hybrid or synthetic varieties. Such varieties have been most
commonly used in cross-pollinated and often cross-pollinated crop
species. In several self-pollinated species also, hybrid varieties have
been commercially used. Attempts have been made and are being,
made to utilize heterosis in breeding of wheat and barley. But so
far, these attempts have not been successful The chief difficulty m
commercial exploitation of heterosis is in the production of large
quantities of hybrid seed. . This is particularly difficult in those self-
pollinated species where the number of seeds per fruit is small, e.g.,
cereals. Some of the species where heterosis has been commercially
exploited are listed below.
Crop species : A sexually propagated species, cross-pollinated
species, e.g., maize, jo war, bajra, sugarbeets, sun-
flower, forage grasses, forage legumes, and cotton*
and some self-pollinated crops, e.g. 9 rice (in China).
Vegetable crops ; Tomato, brinjal {Solatium melongena), onion ,
cucurbits, Brussels sprouts etc.
Fruit Trees : In almost all the fruit" trees.
Animals : . Silk worm, poultry, cattle, swine.
This list is by no means comprehensive and many other
■examples may be found in the literature. In India, heterosis has
been commercially exploited in maize, jowar, bajra, cotton, asexualfy
propagated crop species and fruit trees. . ' 7
m
Piani Breeding : Principles and Methods
SUMMARY
Ira cross- pollinated species* inbreeding leads to loss in vigour and fertility ;
j« known as inbreeding depression. There is a general reduction in size of
Various parts and in yield. In many species, harmful recessive alleles appear
in varying frequencies. Many lines may be lose due to severe reduction fa
vigour and fertility. The degree of Inbreeding depression vanes considerably
from one species to another. Some species, onion* cucurbits etc.* show little
oir no inbreeding depression ;■ in species like maize and bajra there is moderate
inbreeding depression, while in Some species, such as alfalfa . and carrot, the
inbreeding depression is very severe. Self- pollinated species* on the other
hand, are adapted to inbreeding and do not show any inbreeding depression.
fi hybrids from two unrelated genotypes are often more vigorous and
fertile than the parents this h known as heterosis. Heterosis is generally ex-
oressed •»$ an increase in size and yield, hut -it may be expressed in several
other wavs, e.g , earlier or later flowering and maturity, greater adaptability,
faster growth* rate etc. Heterosis is observed both in cross- and. self-DOlIinated
specks " There are two genetic hypotheses to explain heterosis : dominance
bvpothesis and ovcrdominancc hypothesis. Dominance hypothesis states that
heterosh is the result of masking of harmful effects of recessive aiieles by their
dominant alkies. According to overdominance hypothesis, heterosis is the
result of superiority of the hetcrozygotes. There is no conclusive evidence to
support or to reject, either of the two hypotheses. Heterozygotes at certain
loci are superior to homozygotes, h is generally believed that heterosis is the
result of dominance andepi static interactions ; some genes may also show over -
dominance .
At the biochemical level, heterosis may result in one of many ways.
Some of the mechanisms know n so far are : reduced amount of a single gene
oroduct, separate gene products by the two alleles, combined gene product or
the hybrid substance and effects in. two different tissues. Heterosis is com-
mercially exploited in form of hybrid varieties or synthetic varieties.
Heterosis has been extensively used in many crop species, vegetables, fruit trees
and in animals.
QUESTIONS
1. Define inbreeding depression and hybrid vigour. How do these pheno-
mena affect the seif- and cross-pollinated species ? Explain with the help
of suitable examples.
2. Differentiate between the following ; (i) homozygous^ and homogeneous
lines, (ii) homozygous and heterozygous balance, (iii) genetic ‘oad and
gene pool, < iv) heterosis and luxuriance, (v) heterosis and hybrid vigour,
(vi) dominance and overdominance.
3. What arc the mam features of inbreeding depressipn ? Briefly describe
the degrees of inbreeding depression encountered in crop species.
4. Define heterosis. List its characteristic features, applications and achieve-
ments in crop improvement.
5. Explain the dominance and overdominance hypotheses of heterosis giving
their main features, objections and answers to the objections. Which of
the above two hypotheses is more widely accepted and why ?
6. Write short notes on the following : (i) sickle cell anaemia, (ii) pab locus
in Neurospora , (iii) ma gene in maize, (iv) Adh alleles in maize, (v) inbreed-
ing depression, and (vi) hybrid vigour.
7. Discuss the physiological bases of heterosis.
Heterosis and Inbreeding Depression
193
Suggested Further Reading
Allard, R.W. 1960. Principles of Plant Breeding. John Wiley and Sons, Inc.,
New York.
Gowen, J.W. (ed.). 1952. Heterosis. Iowa State College, Ames, Iowa, U.S.A.
Janossy, A. and Lupton, F.G.H. (eds.). 1976. Heterosis in Plant Breeding.
Elsevier, Amsterdam.
Pal, B.P. and Sikka, S.M. 1956. Exploitation of hybird vigour in the improve-
ment of crop plants, fruits and vegetables. Indian J. Genet, lb : 95-193.
SrNGH, H.B. 1962. Exploitation of hybrid vigour in vegetables. ICAR Series
No. 33.
Sinha, S.K. and Khanna, R. 1975. Physiological, biochemical and genetic
bases of heterosis. Adv. Agron. 27 : 123-174.
CHAPTER 11
Mass Selection
In mass selection a large number of plants , of similar phenotype
are selected and their seeds are mixed together to constitute a new
variety. The plants are selected on the basis of their appearance or
phenotype. Therefore, selection is done for easily observable charac-
ters like plant height, ear type, grain colour, grain size, disease
resistance, tillering ability, lodging resistance, shattering resistance
etc. Sometimes, yield of the plant may be used as a criterion of
selection. If the population has variation for grain characteristics
like seed colour and seed size, selection may be done for them before
the seeds of selected plants are mixed together.
The population obtained from the selected plants would be
more uniform than the original population. It may be expected to
be more or less uniform for easily observable characters, like, plant
height, seed colour, presence of awns etc., governed by one or few
major genes. However, the population is likely to show a consider-
able variation for quantitative characteristics like yield, quality,
adaptability etc. The original population would have been a mixture
of several purelines, and the plants selected from it would be homo-
zygous. Since several plants are selected and their seeds are mixed,
the selected population would be a mixture of several similar looking
purelines. Thus a variety developed through mass selection would
have considerable genetic variation, and consequently further mass
selection or pureline selection may be done in such a variety at a
later date.
Generally, the plants selected in mass selection are not subjected
to progeny test. But Allard (I960) maintains that progeny test should
be done. In such a case, poor and defective progenies are rejected.
The remaining progenies are mixed together to constitute the new
variety.
APPLICATIONS OF MASS SELECTION
In case of self-pollinated crops, mass selection has two major
'Mass Selection
195
applications : (!) improvement of Desi or local varieties, and
(2) purification of the existing pureline varieties.
Improvement of Local Varieties. Mass selection is useful for the
improvement of land, desi or local varieties of self-pollinated crops.
The local varieties are mixtures of several genotypes which may
d,frer in flowering or maturity time, disease resistance, plant height
etc. Many of these plant types would be inferior and low yeilding.
As a result, they would lower the performance of the local variety
Hence the elimination of poor plant types through mass selection
would improve the performance and uniformity of the variety The
. local varieties have been under cultivation for a long time. There-
fore, they would generally be well adapted to the local environment
and would be relatively stable in their performance. Mass selec ion
would improve he variety without adversely affecting its adapts
bd.ty and stabil.ty because the new variety would be made up of
most of the superior plant tv.vs pretent in the original local variety
Purification of Existing Pureline Varieties. The other application of
mass selection is to maintain the purity of existing pureline
varieties. Purelines tend to become variable with time due to
mechanical mixtures, natural hybridization and mutation * is
therefore, necessary that the purity of pureline varieties he
maintained through regular mass selection. At present mass selec
tion is used precisely for this purpose. present, mass selec-
Thus as a breeding method mass selection has only a limited
application for the improvement of self-pollinated crops. It isTeS
rally not used for the handling of segregating populations derived
from hybridization. Sometimes, many phenotypically rimilar oure
ines differing ,n characters like disease resistance are mS toemher
•ml evaluated for their performance 6tforeS^"!^!IjS3 ) '' , SS e4
may also be evaluated for their performance as a mTxSre of the
purelmes Mass selection has also been used for th? setectio^ nf
desirable traits m segregating populations, e.g., for seed size in
But this application is ot limited scope where mass selection ?
concerned character is rather easy and avoidl extenSe 1? for „ the
tion necessary for single plant selection, e.g., for davs to .n.nf'f'
seed size etc. ° iUl oa y s maturity,
has tej’ ^y r imlonm°h sSSSf '“if 3 selection
since i, leads 1 ,o K s i„
several plants arc selecied aJd ihefrseed'are mirM 1 ?,,"??
Ihe nea, generation, inbreeding! avotded or Ite'f o®, raise
Further, because of the heterozveom nature £#■ iu a
several cycles of mass selection may effectiveh^ hf
196
Plant Breeding : Principles and Methods-
the procedure of mass selection
The general procedure for mass selection in developing a new
variety is outlined below and schematically presented in Fig. 11.1.
But the procedure may be modified by the breeder according to the-
needs.
FIRST
YEAR
SECOND
YEAR
oooooooo
oo o o oo oo
oooooooo
oooooooo
□□□□
(i) From a variable population, select
200-2000 plants with similar but
desirable traits
(ii) Composite tbe seeds from selected
plants
(i) Composite seed planted in a pre-
liminary yieid trial alongwith
standard checks
(ii) Phenotype of the selected popu-
lation is critically evaluated
f
r
(0 Promising selections evaluated in
coordinated yield trials at several
locations
(ii) If outstanding, released as a new
variety
J
Seed multiplication for distribution
Fig, 11.1/ Mass selection in self-pollinated crops ate used to develop new
varieties. For maintaining the purity, of pureline varieties,
operations of the first year may be repeated every year’ or after
every few 7 years.
First year. A large number of phenotypically similar plants are
selected for their vigour, plant type, disease resistance and other
desirable characteristics. The number of plants selected may vary
from few hundred to few thousand. If too many plants are selected,
the improvement is likely to be small. But if too few plants are
selected, the adaptation of the variety may become poor. Seeds
from the selected plants are composited to. raise the next generation
Mass Selection
197
Second Year. The composite seed is planted in a preliminary yield
trial along with standard varieties as checks. The variety from which
the selection was made should also be included as a check to deter-
mine if there has been an improvement due to selection. Phenotypic
characteristics of the new variety are critically observed.
FIRST
YEAR
THIRD
year
OCOO O O O O
OOOOO O OO
GO OOOOOO
o.ooooc o o
SECOND IN!
YEAR
IIMIII1I1
□ □□□
0) Select 200-2000 plants of similar but
superior phenotype
0/) Harvest seeds separately from each
selected plant
(i) Grow individual plant progenies
(ip Reject inferior or segregating progenies
(iii.) Bulk the seed from remaining pro-
genies
(i) Preliminary yield trials from the bulked
seed ; standard checks are included
(ii) If superior, the variety is included in
multilocation yield trials
i
— .I..,
i
i i
!
;
! i ii
i — >
(i) Multilocation coordinated yield trails
00 If superior, released as a new variety
•eighth
YEAR
o o o o o o o
GCOOOOp
O C C O G C O
oooococ;
JO O O O G C |
Seed multiplication for distribution
Jug. 11.2. Mass selection in self pollinated crops coupled with progeny
testing. This method is more useful than the one outlined before,
it is commonly used for maintaining the purity of pureline
varieties. F - ^
198
Plant Breeding : Principles and Methods-
Third to Sixth Year. The variety is evaluated in coordinated yield'
trials at several locations. This is done to. test the performance of
the new variety at different locations within an agroclimatic zone.
First, the variety is evaluated in an initial evaluation trial for one
year. If the variety is promising, it is promoted to uniformity
regional trials for two or more years. If promising, the variety will
be identified* for release (see chapter 27 for more detail).
Seventh Year. The variety may be released for cultivation if found
suitable and if recommended by the Central or State Variety Release
Committee.
Two variations of mass selection are given below to serve two
different objectives. In improving a local variety (Fig. 11.2), a large
number of plants are selected and their seeds are harvested separa-
tely. The number of plants may range from several hundred to few
(2 or 3) thousand. In the second year, individual plant progenies
are grown for progeny test. Poor, weak and defective progenies are
rejected. Progenies from heterozygous plants are also discarded.
Care should be taken not to discard more than 20-25 per cent of the
progenies. Seeds from the remaining progenies are mixed together
to make-up the new variety. In the third and fourth years, the
variety is subjected to yield trials in different environments Co check
its performance and adaptability. The new variety is quickly
released since it would retain the adaptation of the originial variety
with a superior performance. Thus prolonged testing is not
necessary; this reduces the cost and the time required for developing:
the new variety.
The second variation of mass selection is used for the purifica-
tion of existing pureline varieties . In the first year, 2-3 hundred or
more plants are selected and harvested separately from the pureline-
to be purified. The plants selected are those typical for the variety.
In the second year, progenies from the selected plants are grown and
observed critically. Progenies which are different from the typical
features of the variety are rejected. The remaining progenies are
mixed together to constitute the concerned variety. This process
may be repeated every few years to keep the variety pure or as often
as it is found necessary. This procedure is followed to produce the
basic seed of purline varieties ; the basic seed is used for the produc-
tion of breeder seed (see chapter 28).
MERITS OF MASS SELECTION
1. Since a large number of plants are selected, the- adaptation of the
orgina! variety is not changed. It is generally accepted that a
mixture of closely related purelines is more stable in performance-
over different environments than a single pureline. Thus varieties
developed through mass selection are likely to be more widely
adapted than purelines.
2. Often extensive and prolonged yield trials are not necessary. This:>
reduces the time and cost needed for developing a new variety.
Mass Selection
199
3, Mass selection retains considerable genetic variability. Therefore,
another mass selection after few years would be effective in
improving the variety further,
4. It is a less demanding method. The breeder can devote more
time to other breeding programmes,
DEMERITS OF MASS SELECTION
L The varieties developed through mass selection show variation
and are not as uniform as pureline varieties. Therefore, such
varieties are generally less liked than pureline varieties.
2. The improvement through mass selection is generally less than
that through pureline selection. It is because at least some of the
plant progenies which make up the new variety would be poorer
than the best pureline that may be selected from among them.
3. In the absence of progeny test, it is not possible to determine if the
selected plants are homozygous. Even in self-pollinated species,
some degree of cross-pollination does occur. Thus there is some
chance that some of the plants may be heterozygous. .It is also
not known if the phenotypic superiority of the selected plants is
due to the environment or the genotype. As suggested by Allard,
progeny test may be included in mass selection "programmes to
overcome this defect.
4. Due to the popularity of pureline varieties, mass selection is net
commonly used in the improvement of self-pollinated crops. But
it is a quick and convenient method of improving old local
varieties in the areas or crop species where crop improvement
has just begun.
5. Varieties developed by mass selection are- more difficult to
identify than purelines in seed certification programmes.
6. .Mass selection utilizes the variability already present in a
variety or population. Therefore, only those varieties that show
genetic variation can be improved through mass selection. Thus
mass selection is limited by the fact that it cannot generate varia-
bility.
ACHIEVEMENTS
Mass selection, in one form or the other, must have been used
by the prehistoric man to develop the present-day crop species. The
differentiation of cultivated species from the wild forms clearly
shows the efficacy and the contribution of mass selection to crop
improvement. Mass selection was extensively used by farmers and
agriculturists for the improvement of self-pollinated crops before
pureline selection came into practice. Subsequently, it was com-
pletely replaced by pureline selection as a method of crop improve-
ment '
At present, the use of mass selection is limited to the purifica-
tion of pureline varieties of self-pollinated crops. Mass selection is
200
Plant Breeding : Principles and Methods
routinely practised every year or every few years to maintain the
purity of pipeline varieties. The contribution of mass selection in
this regard would become much more obvious when one considers
that pureline varieties tend to become genetically variable in a few
years due to mechanical mixture, mutation etc. Thus the superiority
of purelines would be lost quickly if their purity was not maintained
through mass selection. All the available pureline varieties in self-
pollinated, crops are maintained through mass selection.
SUMMARY
Mass selection consists of selection of few hundred to few thousand
pheno typically superior plants whose seeds are composited without or after
progeny testing. The new variety is tested in yield trials before being released.
When a large number of plants are selected, prolonged testing is generally not
i necessary. Mass selection is simple, easy and less demanding. Release of the
new variety requires less time and cost than in the case of purelines. The new
variety is stable in performance and is widely adapted. Mass selection is used
to improve old local varieties or to purify existing varieties or purelines.
Progress under mass selection is generally small, the variety developed is not
as uniform as a pureline, and when progeny test is not done the breeding value
of the selected plants is not known.
QUESTIONS
1. Define mass selection. Describe the procedure of mass selection and discuss
its merits and demerits.
2. Describe the applications of mass selection. Describe the procedure of
mass selection as applied for maintaining the genetic purity of pureline
varieties.
Suggested Further Reading
Allard, R.W. i960. Principles of plant Breeding. John Wiley and Sons, Inc,
New York.
Frey, KJ. 1967. Mass selection for seed width in oat populations. Euphytica
16 : 341 - 349 .
Geadelmann, J.L. and Frey, KJ. 1975. Direct and indirect mass selection for
grain yield in bulk oat populations. Crop. Sci. 15 : 490*494.
CHAPTER 12
Pureline Selection
A pureline is the progeny of a single homozygous , self pollinated
plant. As a result, all the individuals within a pureline have identi-
cal genotype, and any' variation within a pureline is solely due to the
environment. In a self-pollinated species, all the plants are expected
to be homozygous because of continued self-fertilization. Thus
progeny of a single plant from a population of a self-pollinated crop
would be a pureline. In pureline selection , a large number of plants
are selected from a self pollinated crop and are harvested individually ;
their individual progenies are evaluated, and the best progeny is
released as a pureline variety. Therefore, pureline selection is also
known as individual plant selection. A pureline variety is a variety
obtained from a single homozygous plant of a self-pollinated crop.
Apparently, a variety developed through pureline selection would be
a pureline variety, while that developed through mass selection
would be a mixture of several purelines. In self-pollinated crops,
pureline varieties are far more common than mixtures of purelines.
CHARACTERISTICS OF PURELINES
L All the plants within a pureline have the same genotype as the
plant from which the pureline was derived. This is because the
parent plant was homozygous and self- fertilized.
2. The variation within a pureline is environmental and nonheritable .
Since all the plants within a pureline are of identical genotype,
the variation within a pureline is due to the environment only*
Therefore, selection within a pureline will generally not be
effective.
3. Purelines become genetically variable with time . The genetic
variation is produced by mechanical mixtures, natural hybridiza-
tion or mutation. Mechanical mixture and natural hybridization
can he prevented by careful handling of purelines. However,
mutations would produce genetic variability in due course of
time. Man can do nothing about it, except that he may attempt
to maintain the purity of purelines through mass selection.
202
Plant Breeding : Principles and Methods
USES OF PURELINES
h As A Variety. A superior pureline may be used as a variety.
Almost all the present varieties of self-pollinated crops are purelines.
2. As Parents in A Hybridization Programme. A pureline that is
not suitable for release as a variety may serve as a parent in the
development of new varieties. Hybridization programmes are
invariably based on purelines ; some of these purelines may not be
suitable for use as commercial varieties.
3. In Studies on Mutation. In studies on spontaneous or induced
mutations, particularly those affecting quantitative characters, pure-
lines have to be used. This is because any genetic variation arising,
In a pureline would be due to mutation it care has been taken to
avoid mechanical mixture and natural hybridization.
4. Other Studies. In many biological investigations* such as*
medicine, immunology, physiology, biochemistry, nutrition etc., use
of highly inbred lines (virtually purelines) of mice, guinea pigs*
rabbits etc. is essential. This is done to avoid genetic variation in
the experimental material so that the effects of the treatments are
easily detected. And furthermore, the effects of treatment are
not confused with those due to genetic variation in the experimental
material.
HISTORY OF PURELINE SELECTION
Pureline selection dates back to mid-nineteenth century or even
earlier. Between 1840 and 1860, several workers, notably Le Couteur,
Shireff, Haliet and Vilmorins, practised individual plant selections in
crops like wheat (T. aestivum), barley (H. vulgare) and sugar beets
(B. vulgaris ). Loui de Vilmorm proposed the Vilmorin Principle or
Vilmorin Isolation Principle , which is the basis for progeny test- The
Svalof Experiment Station of the Swedish Seed Association was esta-
blished m 1806. The pureline selection (or individual plant selection)
was refined to its present form by the workers at this station. The
genetic basis of purelines was explained subsequently by Johannsen
in 1903. Pureline selection has been the most commonly used
method of improvement of seif-pollinated crops. Almost all the
present day varieties of self-pollinated crops are purelines.
APPLICATIONS OF PURELINE SELECTION
Pureline selection has several applications in the improvement
of self-pollinated crops. It is used to improve local or desi varieties,
old pureline varieties and introduced varieties.
Improvement of Local Varieties. Pureline selection is the favourite
method for the improvement of local varieties which have consider-
able genetic variability. A large number of varieties have been
Pureline Selection 205
developed by this method. Some examples are NP 4 and NP 52
wheat {T. aestivum), NP 11 and NP 12 linseed (L. usitatissimum),
T 1 cowpea (V. anguiculata ) and T 1 mung (V. radiata) ; these are
pureline selections from old local varieties. Pusa Sawani bhindi
(okra) is a selection from a variety from Bihar ; it shows an accept-
able level of field resistance to yellow mosaic virus.
Pureline Selection in Introduced Varieties. Introduced materials are
often subjected to pureline selection to develop suitable varieties.
Several varieties have been selected from introduced varieties.
Shining Mung 1 is a pureline selection from an introduced variety
Kulu Type 1, and PS 16 is a selection from an introduction from
Iran. Kalyaa Sona is a leaf rust resistant selection from an intro-
duction from CIMMYT, Mexico. For other examples of such an
application of pureline selection see Chapter 2.
Improvement of Old Pureline Varieties. Purelines become variable
with time. Often selection is used to isolate new varieties from such
genetically variable purelines. Some examples of pureline varieties
isolated from old pureline are : Chafa (from No. 816) gram; Jalgaon
781 (from China Mung 781), Khargone 1 (from K 119-56), Pusa
Baisakhi (from T 44), CO 2 (from PLS 365/3) and CO 3 (from
PLS 367) mung ; and CO 2 (from P 160), CQ 3 (from PLS 364) and
B 76 (from No. 1766) urid (Vigna mungo). Sometimes off- type plants
appear in purelines. Such off-type plants may be selected to develop
new pureline varieties. A dwarf off type was selected from the tall
pureline variety of rice Kalimoonch 64, and released as a new pure-
line variety Shyama.
Selections for A New Characteristic in A Pureline. Sometimes an
opportunity for pureline selection arises when there is a need to select
for some new character which was not previously important Such a
character may be resistance to a disease, plant type, grain type etc.
An example of such a selection is the isolation of jo war (S. bicolor )
variety resistant to root rot caused by Periconia circinati. Sorghum
variety Dwarf Yellow Milo was not bred for resistance to this
disease. In 1926, this disease became widespread in Kansas, U.S.A.
Resistant plants were isolated from a root rot infested field to deve-
lop a new root rot resistant variety.
Selection in The Segregating Generations from Crosses. Purelines are
generally selected from the segregating generations of crosses. Pure-
lines in such cases may be selected using pedigree, bulk or backcross
method. These methods will be discussed in more detail in Chapters
A GENERAL PROCEDURE FOR PURELINE SELECTION
A general procedure for pureline selection is described below
(Fig. 12.1). The number of years required for any step in the proce-
dure and the procedure itself may be modified according to the need.
The purpose of this description is to serve as a general guide for the
/
(i) Replicated yield trials conducted at
several locations
(ii) Inferior progenies are rejected
(iii) Disease resistance and quality tests
are done
MULTILOCATION
TRIALS
1204
Plant Breeding : Principles and Methods
student. The pureline has three steps : (1) selection of individual
plants from a local variety or some other mixed population, (2)
visual evaluation of individual plant progenies, and (3) yield trials.
FIRST
YEAR
oooooo oo
oooooo oo
OOOO o o oo
QOOOOOOO
SECOND
year
INDIVIDUAL plant
PROGENIES
200-3000 plants are selected on the basis
of their phenotype
DESI OR OLD VARIETY
(A MIXTURE OF PURc-LINESl
(i) Individual plant progenies are grown
(ii) Undersirable progenies are rejected
(MAY RE REPEATED IF NECESSARY)
£!GhTh
YEAR
(0 Best Progeny released as a new
variety
(ii) Seed is multiplied for distribution
SLED f/UL T iPl.CA TiON
Fig. 12.1. Schematic representation of pureline selection in self-
pollinated crops.
<fourt h to
seventh
YEARS
THIRD
YEAR
□□□□
(i) Remaining progenies are planted in a
perliminary yield trial
(ii) Inferior progenies are rejected
PRELIMINARY YIELD
TRIAL
If necessary, this process may be repeated for one or more
Step Three x
Third Year. This step consists of a replicated yield trial for a critical
Pureline Selection
205
Step One
First Year. A large number of plants (200-3000) are selected from a
den or local variety or some other mixed population, and their seeds
■are harvested separately. It is desirable to grow the mixed variety
from which selectmn is to be done, with proper spacing so that
individual plants can be observed and evaluated. In case individua
s£ed Ca ^ ldent!fied ’ individual ^ads or stems may be
The number of plants to be selected depends upon the breeder’s
discretion However, lt should be kept in mind that the sefected
plants will be homozygous. Therefore, all the genetic variation ™;n
be between the selected plants, that is, between plant progen es and
there will be no genetic variation within the progenies. "Thus it h-
essentia that superior genotypes must be selected from the mixed
population. The number of plants selected should b? as large as
possible m view of the available time, land, funds and labour When
2-3 thousand plants are selected, they may be picked random lv Rnt
when a smaller number of plants is select^
superior types should be made. selection tor
In any case, it is advisable to select for easily observable
characters, such as flowering and maturity times, disease resistance
presence of awns, plant height etc. If desirable types are not incS
the'progeny! 13 se ect,0D ’ cannot be expected to be present L
Step Two
Second Year. Progenies from individual plants are grown seoaratelv
"e »sx r 2; Th ' obi “ ti,e °t ,his “p <• •» i-asss
the number of plant progenies to be carried to step three This is
because step three requires substantial land, ’labour, time and funds.
The progenies are evaluated visually. Poor, weak and defective
SSTZJ? Progenies a^e discarded un S
they show a great promise. Selection for plant type, plant height
gram type, ear characteristics, flowering and maturity times etc \re
made because these characters are generally more simpTy inherited
than yield. Disease epiphytotics may be created to test the progenies
for disease resistance. The number of orogenies should j?, n e j
as much as possible to facilitate replicated yield trials The superior
progenies are harvested separate’ \ Since each progeny would P be a
pureline, selection within progenies would be useless.
206
Plant Breeding : Principles and Methods
evaluation of the selected progenies. The best variety is used as a
check, and should be planted at regular intervals, e.g. y after every
20-25 progenies, for ease in comparison. If enough seed is available
a preliminary yield trial may be conducted
Selection is made for easily observable characters including
disease resistance. In case of a preliminary yield trial, yield of pro-
genies is also used as a criterion of selection. The main objective
is to further reduce the number of progenies .
Fourth Year. Replicated yield trials are conducted by the breeder
using the best available variety as a check. Observations are recorded
on disease resistance, flowering and maturity times and other
characteristics. Based on these and yield data, superior progenies
are identified. Quality tests may be conducted and used as a
criterion of selection. Each progeny is equivalent to a strain as it is
a pureline. The promising strains are then included in coordinated
yield trials for further evaluation of the strains. ?
Fifth to Eighth Years. The promising strains are evaluated at several
locations alongwith strains from other breeders. The best released
varieties are used as checks.
Ninth Year. The best progeny or strain is released as a new
variety.
# The time required in each step may be more or less than that
outlined in the above scheme depending upon , several factors. For
example, if enough seed is available from the second year, preli-
minary yield trial may be conducted in the third year. This would
save one year. Further, evaluation in coordinated yield trials may
last more than the 4 years indicated above. This would depend upon 4
the performance of the strain in the multiplication trials.
ADVANTAGES OF PURELINE SELECTION
1. Pureline selection achieves the maximum possible improvement
over the original variety. This is because the variety is the
best pureline present in the population.
2. Pureline varieties are . extremely uniform since all the plants
in the variety have the same genotype. Such a uniform variety
is more liked by the farmers and the consumers than a less
uniform variety developed through mass selection.
3. Due to its extreme uniformity, the variety is easily identified in
seed certification programmes.
1 ^ ' hi
'DISADVANTAGES OF PURELINE SELECTION ■
The varieties developed by pureline selection generally do not
have wide adaptation and stability in production possessed- by
the local or desi varieties from which they are develooed.
Pureline Selection
3.
4.
The procedure of pureline selection requires more time, space and
more expensive yield trials than mass selection. P
The upper limit on the improvement is set by the genetic varia-
tion present in the original population. S ana
S ie mSs ed se!ecUon° Thk^ 0 ^ i ™ 6 t0 pure!ine “lection than .
progmSmes V6S hSS tlnie for other breeding
COMPARISON BETWEEN PURELINE SELECTION AND
MASS SELECTION “ UIN
Mass selection and pureline selection differ in the procedure the
product and the improvement gained. The.chief differences be ween
the two me hods of selection are presented in Table 11 . 1 . It would
be seen that the differences between the two methods arise li.f
because of the difference in the number of selected plants included
to produce the new' variety. p nciuaea
ACHIEVEMENTS
Pureline selection was the most extensNetv u <•
method in the early days of crop improvement* work in i nSa^At
that time, genetically variable dferf or local varieS were avail a |i
that offerred excellent opportunities for pureline selection a e
number of improved varieties were developed bv thh imL/ E e
pollinated crops like wheat (T. aestivum),
sativa), pulses, e.g., gram (C. arietinun i), mun» (V radiata) ’ nrM rrz
mungo) etc., oilseeds, e.g., groundnut (A. hypogaea) Yimfedif ti?'
tissimum), self-compatible species of Brassica viz
tori. # «-«*, £„•„). tob a“o
(Gossypium sp.), jovvar (S. bicolor ), jute ( Corchorus sn ) ’ f 0tt n
..bteetc Thooxampto of variety d«,e£,S“ &1Z
!SdSr d the r “ di *“ ■***“ zz S& p SS3SS
Many wheat varieties developed through nurelinr e »w
■oLkSS cSSS^jfcwJj [“ Mcg y i L . W v°a°‘H ? V tf e,ies
varieties developed through pureline selertinn * 4 - * P £, the
selected from a collection from MuzaffLSr \ , was
*ta.d from Chaitali Mupg™ T 9. t’S
208 Want Breeding : Principles and Methods
Naveen, Kulu 4 and T 1 22 are selections from samples collected
from Bareilly, Sindh, Bihar, Kulu Valley and Shillongani, respec-
tively.
Table 12.1. Comparison between pureline and mass selections.
Mass selection
Pureline selection
The new variety is a pureline. The new variety is a mixture of pure-
lines.
The new variety is highly uni- The variety has genetic variation for
form. Id fact, the variation with- quantitative characters, although it is
in a pureline variety is purely relatively uniform in general appear-
environmental. ance.
The selected plants are subjected Progeny test is generally not carried
to progeny test. out.
The variety is generally the best The variety is inferior to the best pure-
pureline present in the original line because most of the purelines
population. The pureline selec- included in it will be inferior to the
tion brings about the greatest best pureline.
improvement over the original
variety.
Generally, a pureline variety is Usually the variety has a wider adap-
expected to have a narrower tation and greater stability than a pure*
adaptation and lower stability in line variety,
performance than a mixture of
purelines.
The plants are selected for their The selected plants have to be similar
desirability. It is not necessary in phehotype since their seeds are
that they should have a similar mixed to make up the new variety,
phenotype.
It is more demanding because If a large number of plants are selected,,
careful progeny tests and yield extensive yield trials are not necessary,
trials have to be conducted. Thus it is less demanding on the
breeder.
Generally, 9-10 years are required Generally, 5-7 years are required to
to develop a new variety. develop a new variety.
Selection within a pureline Selection within a variety developed by
variety is ineffective unless it has mass selection is effective since it has
become genetically variable, genetic variation.
The produce of a pureline variety The produce is generally not uniform
is uniform in quality. since different purelines making up the
variety may differ in the quality of
their gains etc.
The variety is easily identified in The variety is relatively difficult to
seed certification programmes. identify in seed certification pro-
grammes.
Pureline selection is used in self- Mass selection is used in both self- and
pollinated and often cross-polii- cross-pollinated crops,
nated crops.
An introduction of Gossypium hirsutum cotton from U.S.A.
was subjected to mass selection and Dhar war- American variety was
isolated. Dharwar-American was improved by pureline selection to
isolate Gadag 1 (mass selection in Dharwar-American produced the
I'ureune a election
Briefly describe the procedure for pureline select
this raethod. n5e£h ° d crop improvement and :
tlie . , merits and demerits of pureline <
^cctonw.th pureline selections and explain w i
been used more widely than mass selection?
Suggested Further Reading
vJ'f. !960 -. Principles of Plant Breeding. J<
variety Dodahatti Local). Gadag ] was superior to the parent
variety Dharwar-American in yield and other agronomic characters
but was susceptible to leaf blight. Another pureline cotton variety
Bun 107 was isolated from the Buri variety of G. hirsutum Buri
107 represented improvements in staple length, spinning quality
ginning outturn and was resistant to wilt and frost.
Tobacco (N rustlca) variety Harrison Special and culture 40D
were introduced from U.S^A. Pureline selection in Harrison Specif
yielueu HS 9 (Harrison Special 9) and Harrison Special varieties
(both are similar to each other) which yield about 10 per cent more
than the original variety and are more uniform Chatham Z
selected from the culture 40D ; it is superior ThS 9 in cured leaf
colour and is suitable for late planting. rea ,eat
Keliu variety ofbidi tobacco was subjected to pureline «.!«.
tion to isolate varieties Keliu 49 (earlv bv 10 da vs 1 0 nAr t Se 6
yield and better cured leaf colour than Keliu) mid Kehu^So^per
cent more yield than Keliu 49, cured leaf quality comparabK
Oiher pureline selections in tobacco are Surti 20 Lank?27 T K*
T 59, Ramol 43 etc. ’ Lanka2/ < T 23,
SUMMARY
individual proienfes-are tested and rite^est Dros !arSS nil P lber of f ,an!s whose
i* ‘he most common method of deveiopiog a 'variet^Tl^f Va , riety ;
varieties, introductions and oid nureline vnWAiiL V tro ? iocal or desi
desirable off-tvpe and disease resht-int iinAtfvZ eS ’ - ai i? Por { he isolation of
varieties are extremely u var ‘ e H es - Wreline '
become variable with time. Pure lines P a r» J ,1 h P hen °typtcaliy, but they
hybridization programmes and in varimi? a . S varle,ies - parents in
varieties are desirable for their uniformity and^heva 8 StU v’ e ?j The Pureline
certification programmes. Pureiine selection in seed
ment over the original variety Rnt ihie mathod vts tne max. mum improve-
the breeder and a longer time fban ?° re , a «<™i°n from
Pureline varieties are f Iso developed from segregat?ns 'L^v? 1 " 8 ? variety -
between two or more ou relink in inrii/s « i ^ S^nerations of crosses
been developed by the pureline ' selection!’ 3 ° f Varieties have®
Define pureline.
uses.
QUESTIONS
1 various characteristics of purlines and their
CHAPTER 13
Pedigree Method
Mass selection and pureline selection, described in Chapters 11
and 12, are used for the selection of new varieties from mixed popu-
lations that have homozygous plants. These methods cannot be
applied to segregating populations, e..g , F 2 , F a etc., obtained from
crosses. The methods generally used for handling the segregating
generations may be grouped into three categories : (1) pedigree
method, (2) bulk method, and (3) backcross method. Generally, the
objective of these methods is to develop pureline varieties.
Occasionally, the pure lines developed using these methods may be
composited and used as a mixture of purelines.
In the pedigree method, individual plants are selected from P 2
and the subsequent generations,, and their progenies are tested.
During the entire operation, a record of all the parent-offspring
relationships is kept ; this is known as pedigree record. Individual
plant selection is continued till the progenies show no segregation.
At this stage, selection is done among the progenies because there
would be no genetic variation within the progenies.
PEDIGREE RECORD
In pedigree method, a detailed record of the relationship bet-
ween the selected plants and their progenies is maintained. As a
result, each progeny in every generation can be traced back to the
F 2 plant from which it originated. Such a record is known as
pedigree record or simply pedigree. The pedigree may be defined as
a description of the ancestors of an individual and it generally goes
back to some distant ancestor or ancestors in the past . Thus a
pedigree would describe the parents, grandparents great-grand-
parents and so on of an individual and, preferably, the blood relatives
of the ancestors as well. The pedigree is helpful in finding out if
two individuals are related by descent, have a common parent
in their ancestry, and therefore are likely to have some genes in
common.
Pedigree Method
211
MAINTENANCE OF PEDIGREE RECORD
. The pedigree record may be kept in several different ways, but
H should be simple and accurate. Generally, each cross is eiver, «
num er : the first two digits of this number refer to the year in which
the cross was made, and the remaining digits denote the serial
r 0 £ theto \ 9n
sssr of the &&£S2$%
In the first system, the individual plant progenies in each
generation are assigned row numbers corresponding to their location
generations is assigned he ?o» P
previous generation fiom which it. was derived Thus each F
progeny, derived from individual F 2 plants, is given a number corre?
ponding to the row number at which it is located in p
nfthnt ® e,ected fr0 ™ a F ? progeny are identified by the row number
of that progeny. When individual plant progenies are grown iJp
each progeny is also given the row number of the R plot at
nils * ~ v£ £ e KTteWpt ; 0 °;
lions as ooSjbelow "' “ in ,he s “ b!e 1“ e »' genera-
Number
7911-7
7911-7-4
Description
Progeny in the 7th row in the F, plot.
Progeny in the 4th row in the F 4 plot
selected from the progeny in the 7th
row of the F 3 plot.
Progeny in the 14th row in the Fs
plot, selected from the progeny in the
4th row in the F 4 plot.
Progeny in the 3rd row in the F„ plot
selected from the progeny in the 14th
row in the F s plot.
t . c TJ l us ,w h prog f ny can be traced back to the F. pro sen v for
the h 2 plant) from which it originated. But for determining the
previous years Pr ° geny ’ ** h3S t0 COnSu!t £he records °? the
„ . ^ tbe second system, in each generation the selected otants are
" u, " l, ' rs '«•«" individual progenies* each p?„, e „7
.elected plant bears the serial numbers of all the nlartc : n
s'ele^ e° 1 • $ [ elate ^ it by direct descent. Thus the plants
i y 1 ? Ttre given serial numbers * the Fs progenies from theep
g S S c fJ n rrl e SeriSl plan.“ %
P selected from a progeny in F 3 are given the number of that
212
Plant Breeding : Principles and Methods
■ III ion each selected plant form that progeny is also
a serial number! These two numbers make up 'he progeny
progeny
* . ^ .. • - » .. a nt* SK l YY U nuuwv. ^ - tr » '*'
Similarly the plaots selected from a
number in the ty S c,, . c '“‘£ ,, ': r nu mber, i.e., the serial numbers
F4 Progeny are given the pr ^ gen y was drived,
of ‘ he F * w a, number showing the serial number of the plant among
progeny- ™ S system ,s sammansed
below.
Generation
Fs
Number Description
7911-7 Progeny obtained from plant number
7 selected in Fa,
7911-7-4 Progeny from plant number .4, selec-
ted from the F 3 progeny derived from
plant number 7 selected in F a .
7911-7-4-2 Progeny from plant number 2, select-
ed from F 4 progeny derived from
plant number 4 selected from F 3
progeny obtained from plant number
7 selected in F 2 .
79 1 |„7-4-?-S Progeny from plant number 8, selec-
ted from Fg progeny dervied from
pla nt number 2 selected from F 4 pro-
geny of plant number 4 selected from
F 3 progeny of plant number 7 selected
'• < in Fo.
Tn this system, the pedigree of a progeny is immediately known,
, nd one does not have to refer to the previous years’ records. But
! S a "reater chance of error in this system since more numbers
JZ t ‘ be recorded. The breeder may devise his own system of
re-ord keeping. In both the systems, the progenies are assigned a
Afferent serial number when they become homozygous and are included
innrciiminarv yield trials. This serial number is given only to those
homozygous 'lines that are included in preliminary yield trials.
In th p pedigree record, a brief note is made about the distin-
guishing features of each progeny. In keeping a pedigree record,
the following should be kept in mind.
1 Only important characteristics should be recorded. If a large
' ' number of other characters are noted, the record keeping would
become a great burden.
2 Only promising progenies should be included in the record.
Poor progenies may be simply marked ‘discard’. This would give
more time for studying promising progenies.
3 The pedigree record must be accurate. Keeping no record is better
than keeping an inaccurate record which will create confusion.
The pedigree record is often useful in the elimination of some
progenies. In° later generations, eg., F 5 , F 0 etc., if some lines
Pedigree Method
213
originated from the same F 4 or F 3 progeny and are similar, only one
of these lines need be maintained. Further, it may often be possible
to obtain a general idea about the inheritance of characters by
studying the pedigree record.
APPLICATIONS OF PEDIGREE METHOD
The pedigree method is the most commonly used method for
selection from segregating generations of crosses in self-pollinated
crops. This method is often used to correct some specific weak-
nesses of an established variety (combination breeding). Pedigree
method is also useful in selection of new superior recombinant types.
It is almost always hoped that some transgressive segregants would .
be recovered (transgressive breeding ). Thus the method is suitable
for improving specific characteristics, such as, disease resistance,
plant height, maturity time etc., as well as yield and quality charac-
teristics. Even when the objective of the breeder is to correct specific
weaknesses of a variety, he generally expects to improve the yield
and quality as well.
THE PROCEDURE FOR PEDIGREE METHOD
A general outline of the pedigree method is given below (Fig.
12.1). However, the breeder may modify the procedure according
to his specific needs.
Hybridization. The selection of parents to be used in a cross is the
most important step in a breeding programme based on hybridiza-
tion (see, Chapter 6). The selected parents are crossed to produce a
simple or a complex cross. For convenience, we shall refer to the
seed obtained from both simple and complex crosses as the Fj seed.
F x Generation. F l seeds are space-planted so that each F 2 plant
produces the maximum Fa seed. Ordinarily, 15-30 F 2 plants should
produce enough seed for a good F 2 population size.
F 2 Generation. In l\ 2,000-10,000 plants are space- pi anted to faci-
litate selection. About 100-500 plants are selected and their seeds
are harvested separately.
Usually, the F* papulation size should be 10-100' times the
number of I ; 2 plants that are to be selected, that is, the number of
F a progenies it is possible to handle. The number of Fa progenies
may vary from 100 to 500 depending upon the facilities available
and the objectives of the breeding programme. When closely related
varieties are crossed, the number of Fg progenies maintained would
be considerably smaller chan when the parent varieties are unrelated
by descent. When the objective of breeding is to improve yield, to
recover transgressive segregants, or both, a relatively larger number
of F 2 plants, that is, P 3 progenies, would be selected. ;
214
Plant Breeding : Principles and Methods
year
fourth
YE Aft
FIFTH
YEAR
SIXTH
YEAR
Fi
1
f IRS'*" PARENTS |
*
year
p*
t
p»
SECOND ft
—
. ■ YEAR
— j
II!
Selected parents planted in a
crossing block, and crosses made
10-30 seeds space-planted, har-
vested in bulk
(i) 2 , 000 — 10 , Of 0 plants space-
planted . . _
(ii) 100—500 superior plants
selected
(i) Individual plant progenies
space-planted
(ii) Superior plaids selected
1 1 1 1 1 1 j | (i) As in (i) and (ii) in F.
(i) Individual plant progenies
111 planted in multi-row plots
1 (ii) Superior plants selected from
' superior progenies
seventh ?&
YEAR
> 1 1 ■ i it HI (i) As in (i) and (ii) in F 5
|| j HI!! lit (ii) Preliminary yield trial may
1 1 ’ 1 ’ be conducted
EIGHTH
year
W1WTM TO
□ □□□
I
TH1RTUHTH j ; - f
■y ears
FOURTEENTH
YEAR
(i) Preliminary yield trials
(ii) Quality test
(i) Coordinated yield trials
(ii) Disease and quality tests
Seed increase for distribution
Piu *3 1 A simplified schematic representation
of handling the segregating generate
of
pollinated crops.
of the pedigree method
ons from crosses in sell-
Pedigree Method
The selection in F 2 is based on characteristics that are simply
inherited, e.g., plant height, head type, seed colour, disease resis-
tance, presence of awns etc. Selection for vigour is generally useless
because vigour may be due to heterozygosity, environment or GxE
interaction. Selection for characters like yield or quality are gene-
rally ineffective because these characters are greatly affected by the
environment. The value of selection will largely depend upon
the skill of breeder. This is because he has to judge which F 2 plant
would produce superior progeny for characters like yield; this he
cannot determine on the basis of measurements taken on the F 2
plants (why ?). This is the skill that a breeder develops through a
close and deep study of the concerned crop species. It may be
emphasized that the breeder must not select too many f \ plants. He
should select only as many F.> plants as he can handle efficiently in the
subsequent generations with the facilities at his disposal.
Fa Generation, individual plant progenies are space-planted. Each
progeny should have about 30 or more plants. Individual plants
with desirable characteristics are selected, particularly from superior
progenies. Outstanding plants from inferior progenies may also be
selected. Disease and lodging susceptible progenies and progenies
with undesirable characteristics are eliminated. The number of
plants selected in F 3 should be preferably less than the number of
F 3 progenies. If the number of superior progenies is small, the whole
cross may be rejected.
F.i Generation. Individual plant progenies are space-planted. Again
desirable plants are selected mainly from superior progenies. The
number of plants selected in F 4 is generally much- lower than the
number of F 4 progenies. Progenies with defects and undesirable
characteristics are rejected. Progenies are compared visually, and
more .plants are selected from superior progenies. If two or more
progenies coming from the same F 3 progeny are similar and compa-
rable, only one may be saved and the others may be rejected. Thus
the emphasis is on selection of desirable. plants from superior pro-
genies.
F 6 Generation. Individual plant progenies are. generally planted,
according to the recommended commercial seed rate. Often three or
more rows are grown for each .progeny to .facilitate comparison
among progenies.
Many families may have- become reasonably homozygous and
may be harvested in bulk. In families showing' segregation, indivi-
dual plants are selected. The breeder has to visually assess the yield-
ing potential of progenies and reject the inferior ones The number
of progenies must - be reduced to a size manageable in preliminary
yield trials, which is usually 25-100 progenies.'
F g Generation. Individual plant progenies arc planted in multi-row
plots and evaluated visually. Progenies are harvested in bulk since
they would have become almost homozygous. Progenies showing
segregation may be eliminated unless they .are outstanding ; in such
progenies individual plants may be -selected.
BASIS OF SELECTION
In the segregating generations, thousands of plants nave to be
evaluated for the selection of desirable plants. Almost a- ways, U is
not possible for the breeder to take measurements on the various
characters and base the selection on these measurements. Therefore,
the breeder has to base the selection on quick visual evaluation of the
plants. Selection for morphological characters like height, leaf size
and shape, head type, grain colour, presence of awns etc. and tor
certain other easily observable characters like days to flowering,
days to maturity, disease resistance etc., is easy because these
characteristics are easily observed and evaluated visually. Selection
for many of these characters during the segregating generations is
reasonably effective because they are generally governed by one or a
few major genes, and often they have moderate to high hentabiiity.
Although characters like disease resistance are easily observable,
the differentiation betweeo resistant and susceptible plants depends
upon.a uniform attack of the disease on all the plants of a population.
Disease spread is rarely uniform even in a completely susceptible
population, and the intensity of disease outbreak greatly varies
.from year to year. Therefore, the breeder may find it necessary to
create uniform and heavy disease outbreaks by artificial inoculations,
Preliminary yield .rial W* S.Tnfr'iS
preSsSrSmalldScrn yield d.t. from preliminary yield
trial and/or visual evaluation.
_ .. . viplfl trials with 3 or more replications
F 7 Generation. Preliminary y . jj n€S> The progenies are
are conducted to identify fe # P resistance, flowering
evaluated for plant height, lodging n as addkional
t C S=£ ‘Sr advanced ,o .he coo.dina.ed yie,d
trials. . . .. . ,
F. to F„ Generations. The superior lines
and other characteristics would be released as a new y.
F . Generation When a strain is likely to be released as a variety,
fhitree fa usnally multiplies . its seed
tte f se U ed d onhe S new variety will te'nnddplfeS distribution to the
farmers.
Plant Breeding : Principles and Methods
Pedigree Method
m
i.e., artificial epiphytotics (Chapter 20). Similarly, selection for
lodging resistance depends upon the environmental conditions 'that
lead to lodging. Such conditions do not occur every year. The
breeder, therefore, generally has to select for the breaking strength
of .straw and for the force required to uproot the plants. These
characteristics are positively correlated with lodging resistance.
Selection for cold tolerance may have to be based on the ability of
plants to tolerate artificial low temperatures. This may be neces-
sary because the natural temperature may not go down to the
critical level every year.
Selection for quality characters often requires detailed tests.
Each crop will have a different set of quality characteristics depend-
ing upon its use. For example, in case of wheat milling, baking and
chapati making qualities are important, while in barely malting
quality is valuable. Often the basis of quality is complicated and
several tests have to be conducted for its proper evaluation. For
example, baking quality in wheat is determined by the following
tests : percentage of protein and asb, viscosity of flour, mixing time
and water absorption by flour, loaf volume etc. In cases where
quality tests are complex and time taking, quality tests are generally
delayed till F 7 or F s when the number of progenies has been reduced
to a reasonable size. But quality tests have to be simple, quick and
cheap to serve as a useful basis for selection in the segregating gene-
rations. Some quality tests are simple, rapid and inexpensive, and
are widely used, e.g. y determination of brix in sugarcane, of a
cyanogenic glucoside in Sudan grass etc.*
The most discouraging aspect of the breeding methods is that
there is no simple, quick and reliable method of selection for yield-
ing ability in ' the segregating generations, particularly in Fg to F 4
where single plants are selected. This is primarily because yield has
a very low lieritabiiity in the early segregating generations, and
yield of individual plants is generally unreliable in predicting the
yielding ability of their progeny. Space planting, heterozygosity,
environmental influences and GxE interactions all contribute to
the low lieritabiiity of yield. This problem is discussed in more
detail later.
EARLY GENERATION TESTS
Some parents combine well to produce superior progeny, while
others do not. Similarly, some F 2 plants from a cross would be more
likely to produce superior progeny than others. If crosses and plants
likely to produce superior progeny are identified at an early stage, ■
say in F a , Fs or P 4 , much labour, space, time and money would be
saved. Thus the objective of early tests is to select (!) superior
crosses,, and (2) superior plants from within superior crosses.
Selection among Crosses, ' Performance, that is yield, of F 2 , F 3 dr
F 4 bulks of different crosses gives some information about the per-
formance of the progenies derived from them. This information
would be reliable if the yield trials are conducted at more than one
21g plant Breeding : Principles and Methods
location in two or more years. Poor yielding crosses are generally less
likely *to produce better yielding progenies than he higher yielding
ones Early generation bulk tests may, therefore, help the breeder in
discarding the poor crosses and allow him to give more attention
lo the superior crosses. ,
A more reliable information about the potential of crosses may
OTSbe a r n a e bom r! 100 h cr more, of iSSai plant progenies from each
£ta mean yielding ability of the crosses, genotypic variance among
fhe progenies and expected genetic advance under selection may be
MtimScd (Chapters 4-6). A desirable cross should have high mean
vteld high' genetic variance and high expected genetic advance
under selection. It may be stressed that the trial should be conduc-
ted over locations and years, wherever possible, for a reliable
estimate of the genetic variance. The values obtained from trials at
a single location may be misleading. Why >
Although such detailed trials would require time, space, labour
and funds, the elimination of poor crosses may more than compen-
sate for them. This not only saves times and other facilities by
reducing the number of progenies to be handled subsequently but
nrovides the breeder with more time to concentrate on crosses from
which he is more likely to select a superior progeny. Even selection
has°d on a single trial at one location may oe helpful in discarding
the poor crosses. At C 1 MMYT, Mexico, wheat crosses are regularly
evaluated in F.,, and the inferior crosses are rejected. Only superior
crosses are taken to F 3 and inter generations.
Selection within Crosses. Selection based on characteristics of single
plants in F» is effective for characters with high heritabiiity, but it is
ineffective for those characters that have low heritabiiity, e.g,, yield.
The efficiency of selection for yield in F a may possibly be improved
by using some selection schemes. Gardner used a grid method for
effective, selection for yield (3% yield increase/generation for 16
generations) in maize. In this method, the plot in which selection is
to be done is divided into a number of small subplots or grids
(each having 40 plants or so). Selection is carried out in each sub-
plot separately, and no comparison is made among the subplots.
The basis for this method is that the environmental effects within
the smalier subplots will be relatively smaller than those in the entire
plot. Therefore, division of the plot into grids is expected to reduce
the ' environmental influences and increase the effectiveness of
selection. This method has found some application in cross polli-
nated crops, but has not been tried in self-pollinated species.
Another scheme, honeycomb design, has been proposed by Fasouias
to overcome the confusing effects of the environment on individual
plant yields. In this design, individual plants are planted at a distance
sufficient to eliminate all interplant competition, and in a manner
so that thev form hexagons around every plant in the field. The
Pedigree Method
2 19
plant In the centre of a hexagon is compared with all the plants
on the periphery of the hexagon; the central plant is selected only
when it outyields all other plants in the hexagon. The hexagon is
moved successively so that each plant in the field occupies the central
position in one of the hexagons thus formed. The intensity of selec-
tion is varied by changing the size of the hexagon, Le. f having one,
two or three hexagonal rows of plants in a hexagon to yield 143,
53, and 2.7 per cent selection intensities, respectively. Thus the
honeycomb design uses the principle of moving average, but in the .
case of single plant data. There are conflicting reports on the
effectiveness of this design, and there is no convincing evidence in
its favour. In any case, the design (I) requires a special planter
designed for the purpose, (2) considerable precaution in planting
of the experiments and in recording of data are needed, and (3)
much more land is required due to the much larger than the normal
spacing among the plants.
The above techniques (1) involve yield measurements on thou-
sands of individual plants which is time consuming and cumbersome,
and (2) their effectiveness is not clearly established. A relatively
more successful scheme appears to be the indirect selection for yield.
This is done by selecting for an easily measurable character which
has high heritability and exhibits a high positive correlation with
yield. A well known example of such a scheme is the selection for
increased ear number/plant (prolificacy) in maize by Lonnquist and
others. Ear number/plant in maize has a high positive correlation
with yield, is easily observable and measurable, and has high herita-
bility. However, such handy characters are not available in many
crops, and it is doubtful if the same trait will be equally useful in
every population of a crop species, or through several selection cycles
in the same population. Apparently, the breeder has to solely rely
on visual selection based on his skill and judgement for the identifi-
cation of F 2 plants likely to' yield superior Fa progenies since,
unfortunately, there does not appear to be a reliable tool to assist
him in this matter. A good breeder partly develops this skill through
a painstaking and deep study of the crop plant, and partly he has
a natural gift for it. A good breeder generally has the feel of the
type of progeny a plant is likely to produce. It is, therefore, not
surprising that the available evidence indicates that, at leasiin ■
cereals, visual selection in F 2 is as effective as selection based on
yields of individual F 2 plants.
Replicated trials of F 3 or F 4 progenies provide a reasonably
reliable basis for selection among progenies for yield and other
characteristics with low heritability, particularly when the trials are
conducted over locations and years. At the least, such trials should
be conducted at more than one location to be of some value in
selection. The breeder may, however, reject only the poorest yield- '
ing , progenies to be on the safe side. There are two main schemes
for evaluation of the usually large number of Fa ■ progenies selected
from a cross. • These schemes attempt to minimise the effects of soil
nQ plant Breeding : Principles and Methods
heterogeneity and other environmental factors aad .hereby inere.e.
the efficiency of selection. . . . t ,
„ One row or plot of a control variety is planted
Contiguous Control. One P n j es xhis ensures that
after every two rows (or plots) of Fa gjjg"” riet The yields c f
each F 3 progeny is loc& ® oer'cent of that of the control variety;
P 3 progenies are ^ssed as per the ogenies
theSC , V fS S the sSrior F, progenfes. It is expected that the
to identify fitfilitv and other environmental factors will be
variation m soil /crnl y variety. Therefore, the estimates
expressed in the y.eld of the control vamt^ ^ ^ ^ variet
° f ri Sow f coSsiderably reduced variation due to the environmental
Z&StXSS. 'in'such a casc.'fhe "yield f an F, proga „y is
exposed as per cent of that of the nearest check row or plot.
. r me thod Fi progenies are planted in a
Moving Average. In th et "°°; ? i nc i uc |ed in the trial. The
random order, and no is exn rested as pe^ cent of the average yield
yield of an ha progeny p j n question and the nearest
of seven F, yields of the first three and
!Ea-uhS" proi Stet . block are expressed as per cent of the
th 1 “ t of the seven nearest progenies, including the progeny
average yield ottne seven „ e w av | ra „ e yield of more or less
in Cfof'nresrenies for the comparison. Thus for comparison
of each F 3 progeny, a different average is calculated ; this average
f t h the same purpose as the check variety m the contiguous
serves the^sa P P evidence that the contiguous control
and hi movffig ave age methods may be of comparable efficiency ,n
thc idcnSion of superior yielding F, progen.es; the latter, how-
ever has the advantage of requiring less land and other resources as
plots of a control variety are not planted.
The cost and effort needed for the detailed replicated trials
over locations and years may not be justified in view of the limited
nrowess made under selection. It appears that the accepted practice
of sEtion based on easily observable characters and visual evalua-
tion n eariy generations is a sound one, and no alternative approach
of woven effectiveness is available at present. Postponing the yield
trials to F* To Fa appears to be justified in view of the low
heritability of yield in the early generations.
OFF-SEASON CROPS j ,
The time needed for developing a veriety may be reduced if
off-season crops are grown at suitable locations. Fj generation may
be safely advanced to F» in greenhouse or m an oft-season nursery.
Segregating populations may also be advanced in case they are
Pedigree Method
221
bulked. The land and other facilities at off-season nurseries or in a
greenhouse are generally limited. Therefore, it may not be possible
to grow a large number of progenies In them. Off-season nurseries or
greenhouses cannot be used for making selections, except for
simply inherited characters. This is because the environment in these
cases would generally not be comparable to that at the place for
where the variety is being developed. An off-season crop of wheat
(T. acstivum ), barley (//. vulgare ), etc. may be grown at Wellington
(T.N.), of rice (O. saliva) at Cuttack (Orissa), two off-season crops
of maize (Z. mays) at Dholi (Bihar) and Varanasi (U.P.), of pulses
at Sangla (Kinnaur) and Lahaul-Spiti ((H.P.), and of gram (C. arieti-
num) at Taper-Waripore (J.K.).
MERITS OF PEDIGREE METHOD
1. The method gives the maximum opportunity for the breeder to
use his skill and judgement for the selection of plants, parti-
cularly in the early segregating generations.
2. It is well suited for the improvement of characters which can
be easily identified and are simply inherited.
3. Transgressive segregations for yield and other quantitative
characters may be recovered in addition to the improvement in
specific characters.
4. It takes less time than the bulk method to develop a new
variety.
5. The breeder may often be able to obtain information about' the
inheritance of qualitative characters from the pedigree record.
6; Plants and progenies with visible defects and weaknesses are
eliminated at an early stage in the breeding programme.
DEMERITS" OF PEDIGREE METHOD
L Maintenance of accurate pedigree records takes up valuable
time. Sometimes it may be a limiting factor in a large breed-
ing programme.
2. Selection in a large number of progenies in every generation is
laborious and time consuming. The breeder would not be able
to handle many crosses according to the pedigree method.
3. The success of this method largely depends upon the skill of
the breeder. There is no opportunity for natural selection to
influence the populations.
4. Selection for yield In Fs and F 3 is ineffective. If care in not
taken to retain a sufficient number of progenies, valuable geno-
types may be lost in the early segregating generations.
Plant Breeding : Principles and Methoa
ACHIEVEMENTS . . , . . wflc
Once the genetic variability present m desi varieties was
exploit'd, hybridization was used for further improvement in the
crops Pedigree method has been the most extensive!} use,, method
for handling the segregating generations from the crosses. A large
number of varieties have been developed by the pedigree method in
crops like wheat (T. aestivum), rice (O. saliva), barley (i/. viilgare),
nukes oilseeds cotton (Gossypium sp), tobacco (Nicotiana sp.),
fowar (/ E/or), vegetables etc. Nearly all the improved varieties
of self pollinated crops recommended for cultivation have been
developed by the pedigree method. These varieties represent
improvements in yielding ability, quality, disease resistance and other
agronomic characteristics. A few examples are briefly described
“"i** of the NP Wheats NP 52, NP »S, NP l»
NP i ?< 700 and 800 series of NP wheats etc.) were developed by the
pedigree method. K 65 wheat is a tall variety recommended for
rainfed conditions. It was isolated from the cross C 5. £
K68 was developed from the cross NP '73 a K 13 . ’ u 7 ,f
coloured grains and a very high chapat. making quality. WL 7H
was selected from the cross (S 308 x Chris) a Kalyan Sonj It
is a double dwarf variety and is very high yielding, but it is
highly susceptible to Karnal bunt. Malviya 1_ wheat has amber,
hard medium bold grains and is suitable for early as well as ttmdy
conditions, it performs well even at low fertility with
restricted irrigation. Malviya 12 was developed from the cross
NP 876 < Cno 66. , ,
Tn rice breeding, Taichung Native 1 and IR 8 have contributed
directly or indirectly in the development of many high yielding
popular varieties. The cross Taichung Native 2 xT 141 produced
two ouktanding rice varieties Jaya and Padma ; Padma has shor er
duration and finer grains than Jaya. Other varieties developed
through the pedigree method are Bala, Cauvert, Karuna, Krishna,
Ratna, Sabarmati etc.
As noted previously, G. hirsutwn variety Gadag a purelme
selection from Dharwar-American variety, was susceptible to red
leaf blight. To remove this defect, Gadag 1 was crossed to Combodia
Coimbatore -2 (CC 2) which is resistant to red bhght (CC 2 is a pure-
line selection from the Combodia cotton of Madras). From the
cross Gadag 1 x CC 2, variety Laxmi was selected. Laxmi is superior
to Gadag i in ginning outturn, fibre properties, earliness and resis-
tance to leaf blight. ...
Pusa Early Dwarf is a short statured early maturing variety of
tomato It was developed by pedigree selection from the cross
Meerutix Red Cloud. Red Cloud is an early dwarf variety intro-
duced from U.S.A. The fruits of Pusa Early Dwarf are medium
sized and slightly flattish. It gives about 33 per cent more yield in
the first pick and about 25 per cent more total fruit yield than Pusa
Ruby.
Pedigree Method
223
SUMMARY
In pedigree method, individual plants are selected till the progenies be-
come homozygous. In P 5 and onward, selection within families becomes less
and less effective due to increasing homozygosity. A record of all parent-
offspring relationships is maintained so that each paogeny can be traced back
to the Fa plant from which it originated. Selection for easily identifiable
characters with high heritabiiity is effective in Fa and subsequent generations.
But selection for complex characters like >ield that have low heritabiiity is
ineffective. Preliminary yield trial is generally conducted in F 6 or F,. Repli-
cated yield tests are conducted for upto four years at several locations. The
best line is then released as a new variety.
Early generation tests may be used to identify superior crosses. But
identification of superior plants within crosses in F 2 or F 3 is generally not
feasible due to high effects of the environment: on yield. Special techniques are
available to assist in selection for disease resistance, lodging resistances quality
etc. Off-season nurseries may be used to advance a generation, from Fi
to Fa, which would save the time required for developing a new variety.
The pedigree method provides the best opportunity for the breeder tc
exercise his skill in selection. It also provides* opportunity for selection of
transgressive segregants. But the method reouires considerable attention front
the breeder. 1 his limits the number of populations' ho can handle. Keeping of
accurate pedigree records is time consuming, and often it is a burden.
QUESTIONS
1. Write short notes on the following : (i) pedigree record, (ii) early genera-
tion tests, and (iii) off-season nurseries.
2. Briefly describe the procedure of pedigree method of breeding. Discuss the
achievements through this method of breeding.
3. Describe the basis of selection in the segregating generations. Discuss the
merits, demerits and applications of pedigree method.
Suggested Further Readings
Allard, R.W. I960. Principles of Plant Breeding. John Wiley and Sons, Inc.,
New York.
Briggs, K.G. and Sheeeski, L.H. 1971. Early generation selection for yield
and breadmakiog quality of hard red spring wheat Euphytica 20 : 453-463.
Eagles, H.A. and Frey, KJ. 1974. Expected and actual gains in economic
value of oat. lines from five selection methods. Crop Sc*. 14 : 861-864,
Fasoulas, A. 1981. Principles and Methods of Plant Breeding, Ptib.No. il.
Aristotelian University of Thessaiondki, Greece.
CHAPTER 14
' Bulk Method
The bulk method of breeding was first used by Nilsson-Ehle in
1 90S at Svalof. This method is also known as the mass method or
lilt population method of breeding.' In the bulk method , F 2 and
subsequent generations are harvested in mass or as bulks to raise the
next generation. At the end of the bulking period , individual plants
are selected and evaluated in a similar manner as in the pedigree
method of breeding. The duration of bulking may vary from 6-7 to
30 or more generations. During the bulking period, artificial selec-
tion may or may not be practised. The essential difference between
the bulk and the pedigree methods, therefore, lies in the manner in
which the segregating generations are handled ; in the pedigree
method, individual plant progenies are grown and evaluated in F 3
and the subsequent generations, while in the bulk method these
generations are grown as bulks.
APPLICATIONS
The bulk method is suitable for handling the segregating genera-
tions of cereals, smaller millets, most of grain legumes and oilseeds.
It may be used for three different purposes : (1) isolation of homo-
zygous lines, (2) waiting for the opportunity for selection, and (3) to
provide opportunity for natural selection to change the composition
of the population.
Isolation of Homozygous lines. The most common use of the bulk
method is for the isolation of homozygous lines with a minimum of
effort, and expense. The population is carried to F 6 or F 7 as a bulk ;
by this time the population approaches homozygosity. Individual
plants are selected and evaluated as in the pedigree method. Since
the selected plants would be almost homozygous, most of the pro-
genies will be homogeneous. Therefore, a preliminary yield trial may
be planted in the second year after selection of the individual plants.
Bulk Method
?25
Waiting for the Opportunity for Selection. Selection for resistance to
diseases, lodging, cold etc. depends upon the presence of suitable
environmental conditions favouring disease epidemic, severe lodging,
cold-killing etc. Such environments do not occur every year. The
segregating generations may be carried in bulk until such environ-
ments occur. Individual plants are then selected and handled as' in
the pedigree method. The duration of bulking in this case would
depend upon the occurrence of the particular environment ; it may
end in F* itself or may continue upio F 6 or beyond. This method is
generally known as the Mass-Pedigree Method of Harlan .
Opportunity for Natural Selection 0 Maintenance of bulks is inex-
pensive and without much effort. Some bulk populations may be
carried upto Fgo’or F 30 to provide an opportunity for natural selec-
tion to act. In such a case, the bulking period has to be fairly long
because upto F 6 the population would be, to some extent, hetero-
zygous and natural selection would act on heterozygous plants. ’ But
after F 7s natural selection would act on homozygous plants and
would change the frequency of homozygous genotypes present in
the population. It is assumed that natural selection would favour
higher yielding genotypes and eliminate the poorer genotypes. There-
fore, from a population maintained as a bulk for a long period, one
may expect to isolate superior lines at a much higher frequency
than from the F 2 of the same cross. This method was termed as -the
Evolutionary Method of Breeding by Seneson as it allows ' natural
selection’ to act on the population and change its genotypic
composition.
THE PROCEDURE FOR BULK METHOD
The exact procedure for the bulk method would vary depending
upon the objective of breeder. The following procedure is describ-
ed for the isolation of homozygous lines (Fig. 14.1). The breeder
may introduce various modifications in the scheme to suit his needs.
Hybridization. Parents ate selected according to the objective of the
breeding programme. A simple or a complex cross is then made
depending upon the number of parents involved.
Fj Generation. Fi is space-planted and harvested in bulk. The dum-
ber of Fi plants should be as large as possible ; usually more than
20 plants should be grown.
F 2 -F 6 Generation. F 2 to F 6 generations are planted at commercial
seed rates and spacings. These generations are harvested in bulk.
During this period, environmental factors and disease and pest out-
breaks would change, to some extent, the frequencies of the different
genotypes in the population. Artificial selection is .generally -not
done. The population size should be as large as possible, preferably
30,000-50,000 plants in each generation.
SIXTEENTH
YEAR
Seed Increase for distribution
F{g. 14.1. A generalised scheme of bulk method for the isolation of
homozygous lines of self-pollinated crops.
Plant Breeding : Principles and Methods
FIRST
YEAR
Selected parents are hybridized
'R-i.
SECOND
YEAR
THIRD
YEAR
FOURTH
YEAR
FIFTH
YEAR
SIXTH
YEAR
SEVENTH
YEAR
EIGHTH
YEAR
Fi space-planted, seed harvested in
bulk
Ft planted at commercial"]
seed rate, seed harvested 5
in bulk f
As in Fs
As in Fa
As in Fa
As in Fa
May use
.artificial
f selection,
disease
cpiphyto-
cics etc.
F- is space-planted, indi- I
vidual plants selected, !
seed harvested separately J
NINTH
YEAR
TENTH
YEAR
eleventh
TO
FIFTEENTH
YEARS
I
□ □□□
I
F, 0 -
(i) Individual plant progenies grown
(ii) Inferior progenies eliminated
(i) Preliminary yield trials using stan-
dard varieties as checks
(ii) Quality tests done
(i) Multilocation yield trials
Bulk Method
F7 Generation. About 30-50 thousand plants are space-planted.' 1000
to 5000 plants with superior phenotypes are selected and their seed
harvested separately. Selection is based on the phenotype of plants,
grain characteristics, disease reaction, etc.
Fa Generation. Individual plant progenies are grown in single or
multi-row plots. Most of the progenies would be reasonably homo-
zygous and are harvested in bulk. Weak and inferior progenies are
rejected on the basis of visual evaluation. Only 100-300 plant
progenies with desirable characteristics are saved.
Some progenies would show segregation. Such progenies are
generally rejected unless they are of great promise. In promising
progenies, individual plants may be selected ; preliminary yield trial-
will be delayed for one year in such cases.
F 9 Generation. Preliminary yield trial is conducted ; standard com-
mercial varieties are used as checks. The yield is used as a basis for
selection of superior progenies. Quality test may be conducted to
further reject undesirable progenies. The progenies are evaluated
for height, lodging resistance, maturity date, disease resistance and
other important characteristics of the crop species.
Fxo-F 13 Generations. Replicated yield trials are conducted over
several locations using standard commercial varieties as checks.
The lines are evaluated for important characteristics' in addition to
yield, disease resistance and quality. If a line is superior to the stan-
dard varieties in yield trials, it would be released as a new variety.
Fj 4 ‘Generation. The seed of the released variety is increased for
distribution to the cultivators.
ARTIFICIAL SELECTION DURING THE 'BULKING PERIOD
Generally artificial selection is not done during the bulking
period. But the breeder may eliminate the inferior and undesirable
types and thus assist natural selection. In long-term bulks of 20-30
generations, natural selection plays an important role, but in the
short-term bulks of 6-10 generations the effect of natural selection is
likely to be small. Therefore, artificial selection may be desirable
in short-term bulks.
‘Even in long-term bulks, certain desirable characteristics must V
be selected for because they may npt have any selective advantaged;
or may even have a selective disadvantage. For example, s|ed .
colour, presence of awns, seed size etc. may not have a selec^qp
advantage over the undesirable types. Natural selection would favour
' the . genotypes that produce a large number of seeds irrespective
of seed size. Therefore, selection for larger seeds would be desirable. .
Certain desirable characteristics have a poor survival value in mixed
populations. Artificial selection must be practised for such
DURATION OF BULKING
The duration of bulking would depend upon the objective of
bulking. If the objective is isolation of homozygous lines, 6-8
generations of bulking is enough. Incase bulking is done to carry
-She generation till the suitable environment for effective selection
occurs, bulking ends after the occurrence of the critical environment.
But if the objective is to allow natural selection to act on the popula-
tion, long-term bulks of 20-30 generations are desirable. The popu-
lation would have partly heterozygous plants till Fg or F? and only
by Fjo the population may be regarded as a mixture of homozygous
genotypes. Natural selection on heterozygous plants will be of
little importance because such genotypes would segregate to produce
other genotypes. The natural selection becomes important only
when the population has become homozygous. Therefore, for
natural selection to exert some effect on the frequency of genotypes,
the bulking should be continued for some generations beyond Fm
preferably to F 2 o or F 30 ,
NATURAL SELECTION DURING BULKING
The survival of a genotype in a mixture depends upon the
number (but not weight) of seeds produced, and the percentage of
plants produced by these seeds that reach maturity to produce pro-
geny. Theoretically, the best genotype would increase rapidly in a
mixture, while the poorest types would be eliminated. The medium
average types would show an initial increase, followed by a
decrease when the poorest types would have disappeared. The
second best types would begin to decrease only when the poor types
are almost eliminated from the population. Ultimately, only the
best type would remain, but the second best types would be slow to
disappear. This theoretical consideration is complicated by two
Plant Breeding % Principles and Methods
characteristics to eliminate the undesirable lypes.^ A classical example
of this tvpe is the dwarf plant type in rice. The dwarf types are poor
competitors against the tall, leafy types in a mixed stand. There-
fore tall types must be eliminated from the population to maintain
the dwarf characteristics in the population. A similar situation is
encountered in wheat as well. In any case, artificial selection would
help the natural selection in the elimination of weaker, undesirable
and inferior types from the population.
Artificial disease epiph ytotics may be created to favour disease
resistant types. Insect infestation may be created for selection of
insect resistance. Similarly, other techniques may be used to favour
various desirable types. For example, earliness may be favoured by
harvesting the crop when only a portion of the crop is mature.
Larger seed size may be favoured by rejecting seeds smaller than
a particular size ; the smaller seeds may be easily separated by
passing the seeds over sieves of an appropriate size.
Bulk Method
229
factors -.first, the seed production rate of the best type may not be
the same when competing against the poorest, the average and the
second best types ; and second, the environment in different years
may favour different types in place of favouring only one type in all
the years.
Natural selection exerts considerable influence on the composi-
tion of populations. Evidence favouring this conclusion comes from
studies on the survival of genotypes in mixtures and in hybrid popu-
lations. Studies with varietal mixtures show that one or two varieties
become dominant, while the others decrease in frequency. But at
some locations, the change may be slow and relatively more
varieties may be maintained in the mixture. Different varieties may
become dominant at different locations. Generally, the dominant
varieties would be agronomically superior. More importantly,
inferior types are regularly eliminated. But often the ability of
survival in competition may not be related to agronomic superiority.
Agronomically desirable characteristics like date of flowering, date
of maturity, plant height, disease resistance, determinate habit of
growth etc. may not be related to competitive ability. On the other
hand, such characters may be selected against by natural selection,
e.g., dwarf plant types in rice. However, it is obvious that natural
selection is effective in changing the genetic composition of popula-
tions.
Survival in hybrid populations has been measured in terms of
(1) changes in the frequency of specific genes, (2) performance of
hybrid populations, and ( 3 ) frequency of superior progenies isolated
from hybrid populations. Survival of genes in hybrid populations has
been studied in detail by Suneson and coworkers in . the barley
composite cross II. The composite cross II (note that not a complex
cross) is a mixture of equal amounts of seed from 377 Fjs produced
from 218 varieties crossed in all possible combinations, except
one. It was found that agronomically inferior genes, e.g., hooded,
deficient, black lemma, rough awn, are quickly eliminated from the
population. The degree and, in some cases, the direction of change
(e.g., two-row versus six-row) was greatly affected by the location.
Some seemingly neutral genes may be eliminated possibly due to
their linkage with inferior genes.
The yield of composite cross II was studied from F s to F24 and
compared with the standard variety Atlas. The yield of composite
cross II was initially only 67 per cent of that of Atlas. But it incr-
eased gradually, and in F, 4 the composite cross II out-yielded Atlas
by about 36 per cent. Other evidences also suggest that the
performance of bulk hybrid populations improves with the period of
bulking. Thus natural selection tends to increase the frequency of
higher yielding and agronomically superior genotypes in the bulks.
This point was demonstrated by Suneson by isolating and evaluating
lines from the Composite Cross II at different stages during the
bulking period. More superior progenies were isolated from later
generations than from earlier generations. For exanwfe. out of the
230
plant Breeding : Principles and Methods
536 selections in F 12 , none was of good agronomic type as well as
superior in yield to Atlas. In F 20 , two of the 50 progenies isolated
were outstanding agronomically and outyielded Atlas by 37 per
cent And in F M , each one of the 66 selections was superior to
Atlas in yield and of moderate to good agronomic type. Three top-
yielding progenies outyielded Atlas by 56 per cent. Many plant
breeders are skeptical of these findings, which is evident from the
lack of a widespread application of the bulk method. ^ It is only
natural to expect that a breeding method promising an increase of
56% over the best commercial variety, and requiring relatively small
effort should have been extensively used.
It may be concluded that natural seleccion affects considerably
the frequency of genotypes and genes both in varietal mixtures and
in hybrid populations. Natural selection may not always favour the
agronomically desirable types. But the available evidence indicates
that the yield of bulks increases with time, and later generation bulks
yield more number of superior progenies than those in earlier gene-
rations. Thus bulks method of breeding may be helpful in develop-
ing populations that are more likely to yield superior progenies. In
view of this, the breeder may carry some bulk populations for this
purpose in addition to his other breeding programmes. It may be
pointed out that this would not involve much commitment of funds,
labour and time of the breeder.
A modification of the bulk method
A modification of the bulk method based on artificial selection
in F s and the subsequent generations is outlined below.
F a and F s Generations. F a and F s generations are space-planted and
a large number (1,000-5,000) of desirable plants are selected. The
seeds from selected plants are bulked.
F 4 Generation. F 4 is space-planted and 1,000 to 5,000 desirable plants
are selected. Seed from the selected plants is harvested separately.
F 5 Generation. Individual plant progenies 'are grown. Selection
among progenies is based on plant' height, disease resistance, lodging
resistance, maturity date and other agronomic characteristics.
Undesirable and inferior progenies are eliminated. Often only 10-30
per cent of the progenies maybe saved. Seed from each of the
selected progenies is harvested in bulk.
F« Generation. A preliminary yield trial is planted using the bulk
seed from the selected individual plant progenies. Observations are
recorded on agronomic characteristics and yield. Quality tests may
be done on superior progenies.
F- Generation. Superior progenies selected on the basis of the yield
trial are space-planted for further purification. Individual plants are
>
Bulk 'Method . 23 1
selected from the superior progenies, and their seed is harvested
separately.
F s Generation. Individual plant progenies are grown. Inferior pro-
genies and progenies showing segregation are ordinarily ■ rejected
Each selected progeny is harveged in bulk.
F s Generation. Preliminary yield trial is conducted to identify few
superior progenies, for detailed yield tests. Quality testis done to
eliminate undesirable progenies.
Fio-F 13 Generations. Replicated yield trials are conducted at several
locations with standard varieties as checks. The line or lines that
are superior to the standard checks would be released as new
varieties.
F 15 Generations. Seed of the newly released variety is multiplied for
distribution among the farmers.
This modification of the bulk method provides an opportunity
for the breeder to exercise his skill and judgement in selection of
superior plant types in the early generations (F 2 to FJ. At the same
time it does not involve laborious record keeping as in the case of
the pedigree method. Therefore, this modification has the appeal
that it provides for selection in the early segregating generations and
yet it involves less time and labour than the predigree method.
SINGLE-SEED- DESCENT METHOD
Another modification of the bulk method is the single-seed-
descent method, which ' is becoming increasingly popular. In this
method, 'a single seed from each of the one to two thousand F 2
plants is bulked to raise the Fs generation. Similarly, in F 3 and the
subsequent generations one random seed is selected from every plant
present in the population' and planted in bulk to raise the next gene-
ration. This procedure is followed till F 5 or Fe when the plants
would have become nearly homozygous. In Fs or Fs* a large number
(I to 5 hundred) of individual plants are selected and individual
plant progenies are grown in the next generation. Selection is done
mainly among the progenies, and the number of progenies is suffici-
ently reduced to permit replicated trial in the next generation.
Individual plants may be selected only from outstanding families
showing segregation. Thus preliminary yield trials and quality tests
begin in F 7 or Fs and coordinated yield trials in Fs or Fo (Fig, 13.2),
The objective of single-seed-descent method is to rapidly
advance the generations of crosses ; at the end of the scheme, a
random' sample of homozygous or near homozygous genotypes/lines
is obtained. F 2 and subsequent generations are grown at very high
plant densities as vigour of individual plants is not’ important In’
each year, 2-3 generations may be raised using off-season - nurseries
232
Plant Breeding : Principles and Methods
FIRST
CROP
SECONL
CROP
Fi
THIRD
CROP'
Fa
FOURTH
CROP
fz
FIFTH
CROP
fA
SIXTH
CROP
rg
SEVENTH
CHOP
F*
EIGHTH
CROP
Fr
NINTH
CROP
p £>
T ENTH- o% ll C
ELEVENTH 11
OR fourteenth
CROP
TWELFTH
OR
FIFTEENTH
CROP
F«
oss
III
□ □□
Fi space-planted, harvested in bulk
(i) Fa densely planted
(if) From each plant, one random seed
selected and bulked
(i) F 8 densely planted
(ii) From each plant one random seed
selected and bulked
As in Fa (i) -and (ii)
As in Fa (i) and (ii)
(iy F 8 space-planted
(ii) 100-500 plants with desirable charac-
teristics harvested separately
(i) Individual plant progenies grown
(ii) Weak and undesirable progenies
eliminated
(iii) Desirable homozygous progenies
harvested in bulk
(iv) Individual plants may be selected in
outstanding progenies showing segre-
gation
(i) Preliminary yield trial with a suitable
check
(ii) Quality test
Coordinated yield ■ trials,' disease and
quality tests
Seed Increase for distribution to farmers
Fig. 14.2, Schematic representation of single-seed-descent method.
Note : Since more than one generation may be grown in each year, the
number of generations does not necessarily represent the number
of years.
Bulk Method 23i
and greenhouse facilities. The important features of this scheme
are : (1) lack of selection, natural or artificial, till Fs or F 6 when
the population is reasonably homozygous, and (2) raising of F 3 and
later generations from a bulk of one seed from each F 2 and subse-
quent generation plant in order to ensure that each Fs plants
represented equally in the end population. As a result of the specif
and economy, the single-seed-descent scheme is becoming increasingly
popular with the breeders.
The single-seed-descent scheme (1) advances the generation with
the maximum possible speed in a conventional breeding method ;
(2) requires very little space, effort and labour ; (3) makes the best
use of greenhouse and off-season nursery facilities ; and (4) ensures
.that the plants retained in the end population are a random sample
from the F 2 population. However, (1) it does not permit any form
of selection (which is implied in the scheme) during the segregating
generations ; and (2) in each successive generation, the population
size becomes progressively smaller due to poor germination and
death of plants due to diseases, insect pests and accidents. In some
.crops, e.g. 9 pulses, plant loss may be one of the most serious pro-
blems of the scheme.
DEMERITS OF BULK METHOD . ■ -
1. The major disadvant ige of bulk method is that it takes a much
longer time to develop a new variety. Natural selection
MERITS OF BULK METHOD
1 .
2 .
3.
4.
5 .
6 .
7.
8 .
The bulk method is simple, convenient and inexpensive.
Artificial or natural disease epiphytotics, winter killing etc.
eliminate undesirable types and increase the frequency of desi-
rable types. The' Isolation of desirable types thus becomes
much easier;
Natural selection increases the frequency of superior types in
the population. Progenies selected from long-term bulks are
likely to be far superior to those selected from F 2 or short-term
bulks.
Little work and attention is needed in F 2 and the subsequent
generations. The breeder is free to concentrate more on other
breeding projects.
No. pedigree record is to be kept, which saves time and labour.
Since large populations are grown, transgressive segregants are,
more likely to appear and increase due to natural selection.
Thus there is a greater chance of isolation of transgressive
segregants than in the pedigree method.
Artificial selection may be practised to increase the frequency
of desirable types.
It is suitable for studies on the survival of genes and genotypes
in populations.
becomes ImpoiUU oolj r,».. ~~ r -
~ - dS taft
simply . . _ bulks natural selection has little effect on the
composition of ^ations.^ut ^
in Harlan’s Mass-Pedigree Method,
it nrovides little opportunity for the breeder to exercise his
' skiO or judgement in selection. But m the modified bulk
method the breeder has ample opportunity for practising selec-
tion in the early segregating generations.
4 A large number of progenies have to be selected at the end of
the bulking period. .
5 information on the inheritance of characters cannot be obtained
’ w hidi is often available from the pedigree method.
6. In some cases, at least, natural selection may act against the
agronomically desirable types.
ACHIEVEMENTS
In spite of its merits, bulk method of breeding has not been
widely used in crop improvement. The possible reasons for the <ach
of popularity of the bulk method are : (1) the long time required for
natural selection to act (more than 10 generations) and, therefore,
for developing a variety and (2) the lack of opportunity for the
breeder to exercise his skill in the selection of superior plant types
durinu the segregating generations. But the method has oeen used
for studying the survival of genes and genotypes in segregating
populations ; the Composite Cross II in barley is the best known
example.
The method has been used to a limited extent in barley breed-
ing in U S A , and some varieties have originated from bulk popula-
tions Barley varieties Arivat, Beecher, Glacier and Gem originated
In short-term
usefuTfor ' The' isolation ‘of homozygous
objectives as i
skill or judgement
method, the l
COMPARISON BETWEEN BULK AND PEDIGREE
METHODS
The bulk and pedigree methods differ in the method of handl-
ing the segregating generations and in the importance of natural and
artificial selections. " The end products from both the methods are
nureline varieties. The differences between the two methods of
Bulk Method
Table 14.1. Comparison between bulk and pedigree methods
Pedigree method
Bulk method
Individual plants are selected in F« Fa and the subsequent generations
and the subsequent generations are maintained as bulks,
and individual plant progenies are
grown. ' •
Artificial selection, artificial disease Artificial selection, artificial disease
epidemics etc. ■ are an integral epiphytotics etc. may be used to
part of the method. assist natural • selection. In certain
cases, artificial , selection may be
essential.
Natural selection does not play Natural selection determines the
any role in the method. composition of the populations at
the end of the bulking period.
Pedigree records have to be main- No pedigree records are' maintained,
tained which is often time consum-
ing and laborious.
It generally takes 14-15 years to It takes much longer for the develop-
develop a new variety and to meat and release of a variety. The
release it for cultivation. bulk population has lo be maintained
for more than 10 years for natural
selection to act.
Most widely used breeding Used only to a limited extent,
method.
It demands • close attention from It is simple, convenient and Inexpen-
the breeder from F a onward as sive and .does not .require much
individual plant selections have to attention from the breeder during the
he made and pedigree records have period of bulking.
’to be maintained.
The segregating generations are The bulk populations are generally
space-planted to peimit individual planted at commercial planting rates,
plant selection.
The size of population is usually Large populations are grown. This
smaller than that in the case of and natural selection are expected to
bulk method. increase the chances of the recovery
of transgressive segreganfs.
SUMMARY
In the bulk method of breeding, F 2 and the subsequent generations are
harvested in bulk with or without artificial selection . At the end of the bulking
period , individual plants are selected arid their progenies are evaluated as in the
pedigree method. The duration of bulking varies with the objective of the
breeder. Short-term bulks are useful in Isolation of homozygous lines and in
exposing the population to specific environments for effective selection of desir-
able plant types. However, natural selection has little importance in short-term
bulks. In long-term bulks, natural selection changes the frequency of different
genotypes considerably. In general,, inferior and weak genotypes are eliminated
by natural selection. * Rut often a desirable characteristic may be selected
against by natural selection. Long-term bulks are likely to produce more
number of superior progenies than short-term bulks. Artificial selection may
be used to assist natural selection.
• Bulk method is simple, convenient and inexpensive. During early segre-
gating generations, very Mule work and attention is needed which gives the
Plant Breeding : Principles and Methods
236
breeder more time to concenUa.e on
disadvantage of the bulk me hod 15 '^‘''^f^Treederi" unable to exercise
variety than the ped.gree mc ho ,^ n F “^ h f h r ’ The method is less satisfying
his skill and judgemcnt m sdecu ^ th ^ e , occasionally used; the pedigree
ftilrt jAe talk method is suitable for
Seals, smaller millets, grain legumes and oilseeds.
QUESTIONS
1. Briefly describe the bulk method of breeding and its applications. Discuss
its merits and demerits. . . t „ . , c
2. Discuss the role of artificial and natural selection m the bulk method of
breeding. ■ .... . .
3. write short notes on the follosvmg : in\heZik
methoTiivfnafuml section i’n short-term bulks, and (v) achievements
of the bulk method.
Suggested Further Readings
Allard. R.W. i960. Principles of Plant Breeding. John Wiley and Sons., Inc.,
New York.
Atkins, A.E, 1953. Effect of selection upon bulk barely populations. Agron. i.
45:311-314.
Bal, B.S., Suneson, C.A. and Ramage R.T 1959. Genetic shift during 30
generations of natural selection in barley. Agron. E 51 . 555-557.
Hamblin J 1977. Plant breeding interpretation oi the effects of buik breeding
on four populations of beaus (Phascolus vulgaris). Euphytica 26 : 157-168.
Ham,.. IN J and Morton, J. R. 1977. Genetic interpretations of the effects of
bulk breeding on four populations of beans ( Phaseolus vulgaris). Euphytica
26 : 75-83.
Harlan, H.V.and Martini, M.L 1938. The effect of natural selection in a
mixture of barley varieties. J. Agric. Res. 57 : 189- x9J.
Jain, S.K. 1961. Studies on the breeding of sel ^P oli ‘l ia ^^ S^ ea!s ' The
composite cross bulk population method. EupyUea 10 . 31^-324.
Marshall, H.G. 1976. Genetic changes in oat bulk populations under winter
survival stress. Crop Sci. 16 : 9-15.
Suneson, C.A. 1949. Survival of four barley varieties in a mixture Agron. J.
4:459-461.
Suneson, C.A. 1956. An evolutionary plant breeding method. Agron. I. 48 :
188-190.
Suneson, C.A. 1969. Evolutionary plant breeding. Crop Sci. 9 : 119-121.
CHAPTER 15
Backcross Method
A cross between a hybrid (Fi or a segregating generation) and
one of its parents is known as backcross , In the backcross method ,
the hybrid and the progenies in the subsequent generations are repea -
tedly backcrossed to one of the parents . As a result, the genotype of
backcross progeny becomes increasingly similar to that of the parent
to which it is backcrossed. At the end of 6-8 backcrosses, the pro-
geny would be almost identical with the parent used for backcrossing.
The objective of the backcross method is to improve one or two
specific defects of a high yielding variety, which is well adapted 'to
the area and has other desirable characteristics. The characters
lacking in this variety are transferred to it from, a donor parent with-
out changing its genotype, except for the genes being transferred.
Thus the end result of a backcross programme is a well adapted
variety with one or two improved characters.
For example, a wheat variety, e.g. 9 Malviya 12, may become
susceptible to a disease, e.g<, leaf rust, but is an otherwise highly
desirable variety. This variety, known as the recipient parent , is
crossed to a leaf rust resistant variety, the donor parent , e.g., Sparrow.
The Fi hybrid and the progeny in the subsequent generations are
backcrossed to the recipient parent, /.<?., Malviya 12. During the
backcross programme, rust resistance is maintained by selection. At
the end, rust resistant plants from the progeny are self-pollinated and
homozygous rust resistant progenies are isolated. These progenies
will be almost the same as the recipient parent, /.<?., Malviya 12,
except for their rust resistance (as compared to susceptibility to rust
of the recipient variety). Since the recipient parent is repeatedly
used in the backcross programme, it is also known as the recurrent
parent. The donor parent, .on the other hand, is known as the non-
recurrent parent because it is used only once in the breeding pro-
gramme (for producing the Fi hybrid).
REQUIREMENTS OF A BACKCROSS PROGRAMME
For the successful development of a new variety through back-
cross method, the following requirements must be fulfilled.
Intervarietal Transfer of Simply Inherited Characters. Characters
governed by one or two major genes, e.g., disease resistance, seed
colour, plant height etc., are the most suited for transfer through
backcross method from one variety of the same species to another.
Disease resistance has been the most commonly transferred charac-
ter. Mostly such transfers are successful,- but sometimes they may
fail due to a tight linkage between the gene being transferred and
some other undesirable gene.
Intervarietal Transfer of Quantitative Characters. Quantitative
characters may also be transferred from one variety to another if
they have high heritabiiity. The following quantitative characters
have been transferred : earliness, plant height, seed size and shape.
Interspecific Transfer of Simply Inherited Characters. Backcross
method has been used to transfer simply inherited characters, mostly
disease resistance, from related species to a cultivated species. For
example, transfer of resistance to wild fire-black fire from Nicotiana
longi flora to N. tabacum, of leaf and stem rust resistance from
Triticum timopheevil, T. monococcum, Aegilops speltoides and rye
(S. cereale) to T. aestivum, of black-arm resistance from several
Gossypium species to G. hirsutum etc. Interspecific transfer of genes
is easy when the chromosomes of the two species pair regularly.
But often chromosomes of different species are differentiated by
structural changes that reduce pairing between them. As a result, in
interspecific gene transfers often undesirable linked genes are also
transferred alongwith the desirable gene. Another difficulty in inter-
specific transfers is that the gene may not be able to function in the
Backcross , . Method
239
same way in the genetic environment of the new species
(Chapter 25).
Transfer of Cytoplasm. Backcross is used to transfer cytoplasm from
one variety or species to another. The transfer of cytoplasm is
particularly desirable in cases of cytoplasmic male sterility. The
variety or species from which the cytoplasm is to be transferred is
used as the female parent The recurrent parent, the parent to which
the cytoplasm is to be transferred, is used as the male parent in the
original cross and in the backcrosses. After 6*8 backcrosses, the
progeny would have the nuclear genotype of the recurrent parent
and the cytoplasm from the donor parent. For example, transfer of
T. timopheevii cytoplasm to T. aestivum has resulted in male sterlie
lines of T. aestivum .
Transgressive Segregation. Backcross method may be modified to
produce transgressive segregants. Firstly , the F 1 may be backcrossqd
only 1-2 tiroes to the recurrent parent; leaving much heterozygosity
for transgressive segregants to appear. In the second modification,
two or more recurrent parents 'may be used in the backcross pro-
gramme to accumulate genes from them into the backcross progeny.
Such a modification of the backcross would produce a new variety
that would not be exactly like any .one of the recurrent parents.
Production of Isogenic Lines. Isogenic lines are identical in their
genotype, except for one gene. Such lines are useful in studying the
effects of individual genes (and genes tightly linked to them) on yield
and other characteristics. Isogenic lines are easily produced by the
backcross method (Chapter 4).
Germplasm Conversion. In some crops, valuable germplasm can
not be utilized in breeding programmes since these lines are photo-
sensitive and, as a result, do not flower under the prevailing
photoperiods. Such lines may be used as recurrent parents in
separate backcross programmes with a photoiosensitive non-
recurrent parents for the transfer of photoinsensitivity. The lines
derived from these programmes would be photoinsensitive forms
of their recurrent parents. These new lines are called converted
lines and the process is commonly known as germplasm
conversion . Germplasm conversion makes possible the exploitation
of such germplasm which would otherwise be unavailable to the
breeder.
For example, sorghum land-races from Africa are disease
resistant and possess excellent grain quality, but are photosensitive.
Consequently, they fail to flower in India. Some of these land-races
have been converted into photoinsensitive lines at ICRIS4T,
Hyderabad; these converted .lines can now be utilized in different
breeding programmes.
2 ^ 0 . plant Breeding : Principles and Methods
GENETIC CONSEQUENCES OF REPEATED
backcrossing . .
The genetic consequences of repeated backcrossing may be
summarised as follows.
rX-Li 'iLiros% » «‘he h pS g ^ wo„,5
nroduce°i A A and I zl a, that is, 4 homozvgotes and 4 hetero-
HZ ; * u fhe subsequent backcross generations, the propor-
S of heterozygotes would decrease to 50 per cent ot that m
{he previous generation (Fig. 15.1), which is the same as in the
case of selfing. When several genes are considered together,
Sfp?opordo g n of homozygotes for all the genes m . « . tac kerns
generation is given by the . same formula, [2 -l)/2 J , appli
cable to selling. Thus the first effect of repeated backcrossing
t a rapid increase in homozygosity and in the frequency of
homozygotes.
2. In a
backcross progeny Q A A, 4 A a), the
frequency of
tomo Z y”r thelllele from 'the recurrent parent is 4 while
nomozygu v js onl y i. As the number of genes
that are segregating increases, the frequency of the desirable
homoSgofes, i.e., of the recurrent parent type decreases much
more rapidly in the case of selfing than in the case of back-
SS.' Tor » number of genes, this frequency is (I-)” m the
case of backcross progeny, but only (£)" in the selftd progeny .
Thus it is easier to select for a desirable genotype from a
backcross progeny than from a selfed progeny.
When backcrossing is repeated, the frequency of desirable
homozygote increases rapidly (Fig. 15.1) and in the sixth back-
cross progeny, more than 98 per cent of the plants have the
A A genotype. The same is true for the other genes of the
recurrent parent. Therefore, the genes from the nonrecurrent
parent are rapidly replaced by those from the recurrent parent.
As a result, the genotype of the backcross progeny becomes
increasingly similar to that of the recurrent parent. The rate
of this replacement is the same as the rate of increase in the
frequency of the A A genotype. Consequently, after 7 back-
crosses more than 99 per cent of the genes of backcross
progeny would be from the recurrent parent (Table 15.1). This
is expected when no selection is done in the backcross
progenies. But often selection would be done for the plant
type of the recurrent parent. This will result in a faster rate
of return of the backcross progenies to the genotype of the
recurrent parent.
Backer oss Method
241
Fig. 15.1. Effect of repeated backcrosslng on the frequency of homozygotes
and heterozygotes at a single locus Act, (The proportion of AA is
also the proportion of genes from the recurrent parent).
HVBnlO ZATIOISJ NON- RECURRENT RECURRENT
PARENT x PARENT
as / AA
A a X
BCt
(Aa y AA ) X AA
50% 50%
ec 2
( A a
25%
AA )
75 %
X AA
( As AA )
8C3 ■ $ * X AA
12-5% 87-5%
f Aa Aa'\
8C4 ' f 9 X AA
625% 93 75%
( Aa AA ) a a
BCs ' > / 7 X AA
3 - 125 % ' 96 - 875 %
8Cs
.1-562% 98-A38%
X AA
BC 7
( Aa AA )
BCa 0 391% ? 99 309%
242
Plant Breeding : Principles and Methods
T„hle 15 1 Average proportion of genes from the recurrent parent in different
iMJic mo.*. » r r „ nmffpamme.
Average proportion (in per cent) of
genes from the recurrent parent
Fa
50
BCi
75
BCa
87.5
BCs
93.75
bc 4
96.875
bc 6
98.438 •
BC«
99.218
BC,
99.609
BC 8
99.805
BC a
,99.902
BCio
99.951
3 The gene under transfer must be maintained by selection in the
backcross generations. Otherwise it will also be replaced by its
allele from the recurrent parent This gene and the genes
tightly linked to it would thus remain in heterozygous state in
the backcross generations. Therefore, there would be opportu-
nity in each backcross generation for crossing over to \ 'occur
between the gene being transferred and the genes tightly linked
with it. However, when genes are tightly linked (crossing over
0.1 per cent or less), large backcross .populations would have
to be grown to obtain recombinant types. Often tightly
'linked genes may remain together upto 10 or more backcross
generations.
SELECTION OF PARENTS
Jt is clear that the backcross method of breeding changes the
genotype of the recurrent parent only for the gene under transfer .
That is, only the defect of the recurrent parent is removed through
backcross method. But some unexpected changes in one or more
characters may occur due to genes tightly linked with the gene or genes
being transferred. Therefore , the recurrent parent must be the most
popular variety of the area , which has high yielding ability, desirable
quality and high adaptability. In each crop, one or two varieties
dominate, and they are very popular with the cultivators. Such a
variety may have one or two defects. For example, it may be suscep-
tible to a disease or it may have undesirable seed size or colour.
These defects may be removed through the backcross method.
The nonrecurrent parent is selected for the character that is to
be improved in the recurrent parent . The yielding ability'and other
characteristics of the nonrecurrent parent are not important. The
only important factor in its choice is that it must have the character
to be improved in an intense form. The intensity of the character
should preferably be more than that desired in the recurrent parent
Backcross Method
243
hecause some intensity of the character is generally lost durum
pS “ “ W 8 “ et “ tacfc 8ro»i.<l of thiCS
THE PROCEDURE OF BACKCROSS METHOD
The plan of the backcross method would depend upon whether
the gene^bemg transferred is recessive or dominant. The plan for
transfer of a dominant gene is simpler than that for a recessive
.gene.
Transfer of a Dominant Gene.
_ Let us suppose that a high yielding and widely adapted variety
A is susceptible to stem rust. Another variety B is resistant to stem
rust, anddhat resistance, to stem rust is dominant to susceptibility.
A generalised scheme of the backcross programme for the transfer
<Fig U 15 2 ) CS1StanCe fr ° m variety B t0 variety A is given below
Hybridization Variety A is crossed to variety B. Generally, variety
A should be used as the female parent. This would facilitate the
identification or selfed plants, if any.
F a Generation. F x plants are backcrossed to variety A. Since all the
F x plants will be heterozygous for rust resistance, selection for rust
resistance is not necessary.
First Backcross Generation (BC,). Half of the plants would be
resistant and the remaining half would be susceptible to stem rust
Rust resistant plants are selected and backcrossed to variety A BC
plants resistant to rust. may be selected for their resemblance to
variety A as well.
BC 2 -BC 5 Generations. In each backcross generation, segregation
would occur for rust resistance. Rust resistant plants are selected
and backcrossed to the recurrent parent A. Selection for the plant
type of variety A may be practised, particularly in BC 2 and BC S .
BC 6 Generation. On an average, the plants will have 98.4 per cent
genes from variety. A. Rust resistant plants are selected and selfed;
their seeds are harvested separately.
BCg F a Generation. Individual plant progenies from the selfed seeds
of the selected plants are grown. Rust resistant plants similar to the
plant type of variety A are selected. The selected plants are harvested
separately.
BC 6 F 8 Generation. Individual plant progenies are grown. Progenies
homozygous for rust resistance and similar to the plant type of
variety A are harvested in bulk. Several similar progenies are mixed
to constitute the new variety.
Yield Tests. The new variety is tested in a replicated yield trial along
with the variety A as a check. Plant type, date of flowering, date of
maturity, quality etc. are critically evaluated. Ordinarily, the new
Plant Breeding : Principles and Methods*
FIRST
YEAR
SECOND
year
THIRD
YEAR
.‘OURTH
year
;;'i|
FIFTH
year
SIXTH
YEAR
i;|| \ -
fci '
SEVENTH
YEAR
V ;
j ft
eighth
YEAR
! '•> /,,r„
l'
\i ,
jgk
Jgv.v
NINTH
YEAR
SB
TENTH
year
if
ELEVENTH
YEAR
"•ssrr
RR X
/
/ '
f% &f X rr (RECURRENT ARENT A)
/
c 0,
X rr (RECURRENT PARENT A )
^Rr Xrr (RECURRENT PARENT A ;
/
ec*p 3 i i | | | | | | I I I I I I I
t
B is the nonrecurrent
parent* rust resistant but
agronomically undesirable.
A is the recurrent parent*
rust susceptible, agronomi-
cally desirable.
Fi backcrossed with the*
recurrent parent, variety A
(0 Rust resistant plants
similar to variety A selected?
(ii) Plants backcrossed with
recurrent parent A
(i) As in BCi
(i) As in BC S
(i) As in BCi
(i) As in BCi
(i) Rest resistant plants-
seif-poiii nated
(ii) Seeds of selected plants
harvested separately
(i) Individual plant proge-
nies grown
(ii) Selection for rust resis-
tance and plant type of A
(j) Individual plant proge-
nies grown
(ii) Homozygous progenies*
similar to recurrent parent
A harvested and bulked
Replicated yield trial with
the recurrent parent as one'
of the checks
TWELFTH
YEAR
Seed multiplication for
distribution
Fifl 15 2 A generalised scheme?* for the 'transfer of a dominant
8 ‘ d&ue resistance through the.backcross method in a self-pollinated
species.
JBackcross Method
245
variety would be identical to the variety A in performance. Detailed
yield, tests are, therefore, generally not required and the variety may
‘directly be released for cultivation.
Transfer of a Recessive Gene
When rust resistance is due to a recessive gene, all the
backcrosses cannot be made one after the other. After the first
backcross, and after every two backcrosses, F a must be grown to
identify rust resistant plants (Fig. 15.3). The F x and the backcross
progenies are not inoculated with rust because they would be suscep-
tible to rust. Only the F 2 is tested for rust resistance. A generalised
scheme for the transfer of a recessive gene is given below (Fig. 15.3).
Hybridization. The recurrent parent is crossed with the rust resistant
donor parent. The recurrent parent is generally used as the female
parent.
Fj Generation. F 2 plants are backcrossed to the recurrent parent.
BC 2 Generation. All the plants are self-pollinated. Since rust resis-
tance is recessive, all the plants will be rust susceptible. Therefore,
there is no test for rust resistance.
ECi F ‘2 Generation. Plants are inoculated with rust spores. Rust
resistaut plants are selected and backcrossed with the recurrent
parent. Selection is done for the plant type and other characteristics
of the variety A.
BC S Generation.. There is no rust resistance test. Plants are selected
for their resemblance to the recurrent parent A, and backcrossed
with the recurrent parent.
BC S Generation. There is no disease test. The plants are self-
pollinated lo raise F 2 . Selection is usually done for the plant type
of variety A.
BC 3 F 2 Generation. Plants are inoculated with stem rust. Rust
resistant plants resembling variety are selected and backcrossed to
variety A. Selection for plant type of A is generally effective.
BC 4 Generation. There is no rust resistance test. Plants are back-
crossed to variety A.
BCs Generation. There is no rust test. Plants are self-pollinated to
raise F a generation.
BC 5 F 3 Generation. Plants are subjected to rust epidemic. A rigid
•selection is done for rust resistance and for the characteristics of
variety A. Sslfed seeds from the selected plants are harvested
•separately* •
BCsFb Generation. Individual plant progenies' are grown and
subjected to rust epiphytotic. A rigid selection is done for resistance
to stem rust and for the characteristics of variety A. Seeds from
several similar rust resistant homogeneous progenies are mixed to
.constitute the new variety.
Yield Tests. It is the same as in the case of transfer of a dominant
jene*
246
FIRST
* YEAR
NONRECURRENT
PARENT ©
SECOND
YEAR
THRO
.YEAR
FOuRT* •
YEAR
FJFTh
YEAR
Si* TV*
year
SEVENTH
■ year
EiGhTm
YEAR
X
/
Rr X RR
RECURRENT
PARENT A
RR
/
(RECURRENT PARENT
VARIETY A)
SC,
RR,Rr
(VARIETY A )
NINTH ’ ’ ac& RR »
year 6
\
/
eleventh ac 4 F 3
YEAR
twelfth
■. YEAR
THIRTEENTH
YEAR
Plant Breeding : Principles and Methods?
Variety B is rust resistant,
variety A is susceptible but
agronomically desirable
Varieties A' and B crossed*.
B used as the female parent
(I) no rust resistance test
(ii) Fi backcrossed to the-
recurrent parent
0) No rust resistance test
(ii) All plants are self-polli-
nated
(i) Rust resistance test
(if) Resistant plants selected
and (iii) Backcrossed to
Variety A
(i) No rust resistance test
{iii Plants backcrossed to-
Variety A
O’) No rust resistance test
(it) All plants self-pollinated
(i) Rust resistance test
(ii) Resistant plants selected
and (Iii) Backcrossed with,
variety A
(i) No rust resistance test
(ii) Backcrossed to variety A
(i) No rust resistance test
(ii) Ai! plants self-poilinated
(0 Rust, resistance test
(ii) Resistant plants selected
and harvested separately
(i> Individual plant proge-
nies grown
(ii) Progenies similar to
variety A selected and com-
posited
Replicated yield test
alogwifh the variety A as-
check
Seed 'multiplication for
distribution
II
Fig. 15.3. A generalised scheme for the transfer of a recessive gene for disesase
resistance (in this case rust resistance) through the backcivss method*
In a self-pollinated crop.
Backer oss Method
NUMBER OF PLANTS NECESSARY IN THE BACKCROSS
GENERATIONS
According to the above schemes, only a few (about 1.0) seeds
are necessary in each backcross generation for the transfer of a
character governed by a single gene. This population size would
almost certainly have at least one plant with the gene for rust resis-
tance. However,, if the character is governed by two or more genes,
a' larger number of backcross progenies would be required. A larger
size of backcross population is also desirable to permit an effective
selection for the plant type of the recurrent parent, and to increase
the probability of recombination between the genes being transferred
and the genes tightly linked with it. Therefore, more than 50,
preferably 100 or more, plants should be grown in each backcross
generation. In F* and Fs generations, the population size should be
as large as possible.
SELECTION FOR THE CHARACTER BEING-
TRANSFERRED '
A rigid selection for the character being transferred must be
practised during the backcross and the F 2 generations , otherwise the
character is likely to be lost . It is, therefore, essential that the charac-
ter being transferred must have a high heritability. * Although
monogenic characters are the easiest to transfer, the number of genes
is not as important as the heritability of the character under transfer*
It is desirable that the character should be easily identified either
visually or through simple tests. The breeder should try to maintain
the character in an intense form through selection’ Often some
intensity would be lost due to modifying genes in the new genetic
background . Therefore , the nonrecurrent parent should be chosen for
a high intensity of the character to be transferred.
NUMBER OF BACKCROSSES TO BE MADE
In the backcross method, it is essential that the genotype of the
recurrent parent should be recovered except for the gene being trans-
ferred. The recurrent parent is likely to consist of several closely
similar purelines. Therefore, a sufficient number of plants from the
recurrent parent should be used for the backcrosses. This would
make sure that the new variety will have the same genetic composi-
tion" as the recurrent parent.
.Generally, six backcrosses are sufficient to recover the essential
features of the recurrent parent. Selection for the characteristics of
the recurrent parent, particularly in the early backcross generations,
may often have the effect of one or two additional backcrosses.
Thus six backcrosses alongwith selection for the recurrent parent plant
type in the early backcross generations will be effective in recovering
the genotype of the recurrent parent.
TRANSFER OF LEAF RUST RESISTANCE TO
MALVIYA 12 WHEAT
Malviya 12, a high yielding wheat variety, became highly
susceptible to stem and leaf rusts within a couple of years ot its
release. Genes for resistance to these rusts were transferred trom
Sparrow through a backcross programme. Sparrow shows resistance
to all the three rusts under artificial inoculation in the hot spots in
India. In F, (Malviya 12 X Sparrow), 100 plants were grown ; all
of them were backcrossed to Malviya !-. In BC ls moie, than 500
plants were grown. Epidemic condition was created .by planting
infector rows of Agra Local, a wheat variety susceptible to all the
known races of stem, leaf and stripe rusts. The inoculum consisted
of the naturally occurring races of leaf and stem rusts ; no artificial
inoculation was done. One hundred rust resistant plants were selected
and backcrossed to Malviya 12. In each backcross generation, the
plants were handled in the same way as in the BC*. . In BC±, all the
rust resistant plants (about 300) were selfcd, and individual plant
progenies were grown in BC4F2 ; 50 progenies homozygous for rust
resistance were selected and their seeds were bulked to constitute the
rust resistant Malviya 12. Yield trials revealed a slight (ab<xit 4%)
improvement in the yielding ability of the new variety over the
recurrent parent Malviya 12. During the backcross programme,
the sole criterion of selection was rust resistance. One hundred
rust resistant plants were selected in each generation to ensure that
resistance to both the rusts (stem and leaf) was selected for, and that
the resistant plants were not escapes since artificial inoculation was
not used.
TRANSFER OF QUANTITATIVE CHARACTERS
Quantitative characters with moderateto high herifability, such
as. grain size, plant height, flowering or maturity time etc., are
suitable for transfer through the backcross method. Invariably , some
intensity of the character mil be lost during the transfer. Therefore ,
the nonrecurrent parent must have the character in a more intense
form than it is desired in the new variety. After each backcross, the
progeny are selfed and F* of 500-1000 plants is grown. A rigid
selection for the character being transferred is practised in each F a
generation. At the end of the backcross programme, the F a is
handled according to the pedigree method. Several progenies homo*
zygous for the character being transferred and similar to the
recurrent parent in other characteristics are selected and mixed to
make up the new variety. An example of such a transfer is provided
by the medium grained rice variety Calady. Calady was developed
through Four backcrosses of the Fi hybrid between Lady Wright (a
long-grained variety) and Coloro (a short-grained variety) to Coloro.
After each backcross, F 2 was grown and a rigid selection was practis-
ed for grain size.
Plant Breeding : Principles and Methods
249
Backcross Method
TRANSFER OF TWO OR MORE CHARACTERS TO A
SINGLE RECURRENT PARENT
When two or more characters are to be transferred to the same
variety one of the following three approaches may be used.
ohar.o.cm “ b' 3 "the recurrent par™, or
crossing each of the nonrecui icui v larger backcross
.he hybrid •bb’rtrf.Sntat^SiS^ffr.Mtoor* ringle
population won j, ree ding programme may be delayed because
character. Fuither, tne D £ .t,* ejection of all the characters may
,be conditions |o “ toes the Wo gene under transfer may
„°, occur every year. Somettmes, me . and
be linked In such a case. necessary. Some examples of
;S“lUo r uS!?e S.|r^re m S.w f r.h« genes tr * and Sr *
/r 19 and Sr 25, and Lr 26 and Sr 31.
much longer Ebne fo!Te transJof two or more characters.
gSeSioas ’tSThe’prfigr.e method. This approach appears to be
Ihe most suitable of the three methods.
modifications of the backcross method
The backcross method may be modified in various ways to suit
the neS of .Keener. Following are the three common modih-
cations of the backcross method.
Production of F 3 and F 3 F, and F 3 generations me ^
the first and the third backcross^. ^ of the recurrect
parenUs done^nThe F^and^^generatioM. In ^he teckcniss^K^
S5«£S! hsrvs
fourth, fifth and sixth backcrosses of plaSs from the back-
Ss h s pfoSr- -f tV-j Kssa s. « ser
TSSSSi ££& one or two addi-
.tional backcrosses.
250
Plant Breeding : Principles and Methods
Use of Different Recurrent Parents. Often two or more good varie-
ties have quantitative characteristics 'that are ‘ desirable in the new
variety. These varieties may be used as recurrent parents io the
same backcross programme. Each variety is used as a recurrent
parent for one or two backcrosses. The objective of this approach is
to combine in the new variety some good genes from each of the-
recurrent parents with the genes from the nonrecurrent parent *
Noblisation of sugarcane is an outstanding example of this-approach*
Noble canes (S. officinarnm) were first crossed with the Indian canes
(S. barberi). The resulting hybrids were backcrossed to different
varieties of noble canes to develop a large number of commercial'
sugarcane varieties. A similar approach was used for the transfer
of scab resistance in apples and , for the transfer of high vitamin C
content from wild tomato ( Lycopersicon peruviamm) to the culti-
vated tomato (I. esculentum).
Backcross- Pedigree Method. In this method, the hybrid is backcros-
sed 1-2 times to the recurrent parent.. Subsequently, the backcross
progeny are handled according to the pedigree method. This
approach is useful when one of the parents is superior to the other
in several characteristics but the noncurrent parent is not undesirable
agronomically. The superior parent is used as the recurrent -parent
The p?irpose of the one to two backcrosses is to make sure that the
new variety would get a majority of the superior genes from the
recurrent parent. It also leaves enough heterozygosity for transgres-
sive segregants to appear. The varieties developed by this method
must be put in yield trials as those developed by the pedigree method,,
The same holds true when two or more recurrent parents are used
in the backcross programme.
APPLICATION OF THE BACKCROSS METHOD TO
CROSS-POLLINATED CROPS
The backcross method is equally applicable to cross-pollinated
crops. The method is essentially the ' same as in the case of self-
pollinated crops. The only 'difference is that in cross-pollinated crops,
a large 'number of plants (100-300) from the recurrent parent must
be used in each backcross. This is necessary so that the new* variety
has the same genetic constitution as the recurrent parent. For
example, wilt resistance was transferred to alfalfa variety California
Common from the variety Turkestan. Two hundred plants of Cali-
fornia Common were used for each backcross. • The new variety
Caliverde is exactly like California Common except for' its' wilt
resistance.
MERITS OF BACKCROSS METHOD
1. The genotype of the new variety is nearly identical with that of
the recurrent parent, except for the genes transferred. Thus the
outcome of a backcross programme is known beforehand, and.
it can be reproduced any time in the future.
Backer oss Method '
2 It is not necessary to test the variety developed by the back-
cross method in extensive yield tests because the performance
of the recurrent parent is already known. This may save upto-
5 years time and a considerable expense.
3 The backcross programme is not dependent upon environment,
except for that needed for the selection of the character under
transfer. Therefore, off-season murseries and green- houses can
be used to grow 2-3 generations each year . This would drasti-
cally reduce the time required for developing the new variety.
a Much smaller populations are needed in the backcross method
5 Defects, such as, susceptibility to disease, of a well-adapted
' variety can be removed without affecting its performance and
adaptability. Such a variety is often preferred by the fanners
and the industries to an entirely new variety because they know
the recurrent variety well.
6. This is the only method for interspecific gene transfers.
7. It may be modified so that transgressive segregation may occur
for quantitative characters.
DEMERITS OF BACKCROSS METHOD
1. The new variety generally cannot be superior to the recurrent
parent, except for the character that is transferred.
2. Undesirable genes closely linked with the gene being trans-
ferred may also be transmitted to the new variety.
3. Hybridization has to be done for each backcross. This is often
difficult, time taking and costly.
4 Bv the time the backcross programme improves it, _ the
recurrent parent may have been replaced by other varieties
superior in yelding ability and other characteristics.
ACHIEVEMENTS
The backcross method has been widely used in crop improve-
ment. if has been used for transferring genes
one variety to another and from related species, and for the _ trans ei
of cytoplasm. Backcross method is the only breeding method suit-
able for tbe transfer of genes and chromosomes from one speciw 1 1<
another. There are many examples of interspecific t«“sfer of gen^
using the backcross method. ‘Transfer was the ««t commwcia
TeS ss. wss:
Sm-C? 2M ba w"e 24 devl 2 p 5 ed
gSS" th? F, was highly sterile
COMPARISON BETWEEN BACKCROSS AND
PEDIGREE METHODS
The two methods not only diffei - in their procedure and the
system of mating for raising the generations after Fi, thev differ in
Tahiti products as well. These differences are summarised in
Plant Breeding : Principles and Methods
it set few seeds which gave rise to tetraploid plants (perhaps
as a result of backcross to the G. hirsutum parent). Progeny
from the Fi were backcrossed to G. hirsutum varieties Dharwar-
American (first backcross), Combodia Co 2 (second backcross) and
Meade (third backcross). From the backcross progeny, 170 Co 2
and 1 34-Co 2M were selected These varieties had staple length of
JJ" (about 2.7 cm) and were widely cultivated in Gujarat both under
irrigated and raiofed conditions.
The backcross method has been extensively used for the trans-
fer of disease resistance to popular, widely adapted varieties. This
is illustrated by the popular wheat (T. aestivum) variety Kalayan
Sona, which became susceptible to leaf rust. Rust resistance has
been transferred to Kalyan Sona from several diverse sources*
e.g, Robin, Kl, Bluebird, Tobari, Frecor, HS 19 etc., using the
backcross method. Three multiline varieties, namely, KSML 3,
MIKS 1 1 and KML 7406, have been released for cultivation as a
result of the above. Another example is provided by the hybrid bajra
varieties in India. The male sterile line Tift 23A (the female parent
in the hybrids) was highly susceptible to downy mildew leading to a
high susceptibility of the hybrids. Tift 23A was used in a backcross
programme with resistant lines from India and Africa to develop
downy mildew resistant male sterile lines, such as, MS 521, MS 54! A*
• MS 570A etc., which are now being used for producing downy
mildew resistant hybrids.
Characters other than disease resistance have been transferred
by the backcross method. Good examples of this type are available
in cotton. G. herbaceum varieties Vijapla, Vijay, DIgvijay and
Kalyan are some of the cotton varieties developed by the backcross
method. From a variety Broach Load, a mixture of Goghari and
Broach Desi varieties of G. herbaceum , two pureline varieties,
Goghari A- 26 and Broach Desi 8 (BD 8), were selected. Variety
BD 8 was wilt resistant and had a high spinning value, but it had
a low ginning outturn (34%). BD 8 was used as a recurrent parent
in a backcross programme with Goghari A-26 which had a high
ginning outturn (42-47%). High ginning outturn was transferred
from Goghari A-26, the nonrecurrent parent, and a variety Vijay was
developed. Vijay was further improved through the backcross
method ; improved fibre length and early maturity were transferred
from 1027A L-F (nonrecurrent parent). Variety Digvijav resulted
from the above backcross programme.
Backer oss Method
Table 15.2. Comparison between backcro ss and pedigree methods
Pedigree method
253
3.
and the subsequent generations
arc allowed to self-pollinate.
The new variety developed by this
method is different from the parents
jo agronomic and other character-
istics.
The new variety has to be exten-
sively tested before release.
The method aims at improving the
vielding ability and other charac-
teristics of the variety.
It is useful In improving both
qualitative and quantitative char-
acters.
ft is not suitable for gene transfer
from related species and foe Pro-
ducing substitution or addition
lines.
Hybridization is limited to the
production of the Fi generation.
The Fa and the subsequent gene-
rations fare much larger than
those in the backcross method.
The procedure is the same for
both dominant and recessive
genes
Pi and the subsequent generations are
backcrossed to the recurrent parent.
The -new variety is identical with the
recurrent parent, except for toe
character under transfer.
Usually extensive testing is not neces-
sary before release.
The method aims at improving speci-
fic defects of a - well adapted, popular
variety.
Useful for the transfer of both quanti-
tative and qualitative characters
provided they have high heritabihty.
It is the only useful method for gene-
transfers from, related species and tor
producing addition and substitution
"lines.
Hybridization with the recurrent
parent is necessary for producing
every backcross generation.
The backcross generations are small
and usually consist of 20-100 plants
in each generation.
The procedures for the transfer of
dominant and recessive genes are
different.
SUMMARY
in backcross method, the hybrid and Ure new
repeatedly backcrossed to one of the ‘ j »h e for the character
variety is .therefore .the same :as men nor J current paren! _ The recurrent
transferred trom ^e other pa ^’ h yieiding ability and is popular among
parent is a well aaapted variety, hi gn^t^yi ^ ^ o ^ simply inh t d
the cultivators. This variety 'is 1 ? selected for the intensity of the
characters. T h he ."°"I e r C r ed B actooss method is suitable for transfer of both
characters to he transferr • ®®|k prov ided they have moderate to high
qualitative and quantitative enamours p character would be lost durrog
heritably. O^man*. • X S “w Stic background of the recurrent parent.
G rSS: STS
variety, 0 each character the new°varieties
grammes. Finally, they ’are _ _ be mo dified to allow transgressive
thus developed Backcross met 9^. of the backcross method has been
Plant Breeding : Principles and Methods
characters, qualitative characters, interspecific transfer of characters, transfer
of cytoplasm 9 particularly for male sterility, and for the -production of isogenic
lines. The new varieties developed through complete baokcrossing (6-8 back-
crosses) generally do not require extensive yield trials for their release.
1
2 /
3.
4.
:5.
QUESTIONS
Define the following-: recurrent parent, nonrecurrent parent, backcross,
BGiF*. BC a> Bi and Ba. ' "
List the requirements' for a bac&cross programme. Describe we procedure
of backcross method suitable for the transfer of a dominant gene for
disease resistance.
Describe the procedure for backcross method for the transfer of a recessive
gene. Discuss the merits and demerits of the backcross method of
breeding.
Describe the applications of the backcross method and its achievements.
Write short notes on the following :
(0 transfer of quantitative traits through backcross . method, (ii) appli-
cation of backcross method in cross- pollinated species, (iii) selection
in backcross generations, (iv) modifications of backcross method,
(v) number of plants necessary in a backcross generation, and (vi) number
of backcrosses required in gene transfer programmes.
Suggested, Further Reading
Allard, R.W^ i960. Principles of Plant Breeding. John Wiley and Sons, Inc.,
Briggs, F.N. 1959. Backcrossing-its development and present application
Proc. First International Wheat Genetics Symposium, pp. 8-9. P 1 n
'Briggs, F.N. and Allard, R.W. 1953, The current status of backcross method
of p.ant breeding. Agron. J. 45 ; 131-138. wetnod
•Reddy, B.V.S. and Comstock, R.E, 1976. Simulation of the barker™*
3SKK8SWSS5’ *** s '" c “” w » *«S
CHAPTER 16
Other Approaches to Breeding of
Self-Pollinated Crops
The methods of breeding described in chapters 11 to 15 are
'the common methods used for the improvement of self-pollinated
crops. The objective of these methods, except ^ mass selection, is to
develop pureline varieties. These methods either use the variability
already present in the population (mass selection and pureline
selection) or the variability created through hybridization. The Fi
hybrid from a cross is either allowed to self-pollinate (pedigree and
bulk methods) or is backcrossed to the desirable parent (backcross
method). The effect of either of these approaches is a rapid increase
in homozygosity. It is argued that the rapid increase in homozygo-
sity reduces the chances of recovery of desired gene combinations,
particularly when the genes are linked in repulsion phase.
A number of other breeding approaches have been suggested
for the improvement of self-pollinated crops. Many of these ap-
proaches are modifications of the three basic schemes, v/z., pedigree,
bulk and backcross methods. Usually two of these schemes have
been combined to serve some specific purpose, e.g bulk-pedigree
method, backcross-pedigree method etc. Clearly, these are not new
approaches, but in certain situations they may serve some useful
purpose, A discussion of these modifications is beyond the scope of
this book. Three of the more important • and new approaches are
described below. These approaches .are, (!) multiline varieties,
•(2) population approach and (3) hybrid varieties.
MULTILINE VARIETIES
Generally, pureline varieties are highly adapted to a limited
area, but are generally poorly adapted to wider regions. Further,
their performance is not stable from year to year because of changes
256
Plant Breeding : Principles and Methods
ia weather and other environmental factors. Porelines often have'
only one or a few major genes for disease resistance, soch as, rust
resistance, which make them resistant to some races of the pathogen.
New races are continuously produced in many pathogens which may
overcome the resistance present in the ' pureline varieties. For
example, Kaiyan Soaa wheat (T. aestivum) originally resistant to
brown rust (leaf rust), soon became susceptible to the new races of
the pathogen.
To overcome these limitations, particularly the breakdown of
resistance to diseases, it was suggested to develop multiline varieties.
Multiline varieties are mixtures of several purelines of similar height ,
flowering and maturity dates , seed colour and agronomic characteris-
tics, but having different genes for disease resistance . The purelines-
constituting a multiline variety must be compatible, he they should
not reduce the yielding ability of each other when grown in mixture.
The idea of multiline varieties was put forward by Jenson in 1952
for use in cereals. In 1954, Borlaug suggested that several purelines
with different resistance genes should be developed through back-
cross programmes using one recurrent parent. This is done by trans-
ferring disease resistance genes from several donor parents carrying
different resistance genes to a single recurrent parent. Each donor
parent is used in a separate backcross programme so that each Sine
has a different resistance gene or genes. Five to ten of these lines
may be mixed to produce a multiline variety. Which lines are to be
mixed would be determined by the races of the pathogen prevalent
in the area. If a line or lines become susceptible, they would be
replaced by resistant lines. New lines would be developed when new
sources of resistance become available. The breeder should keep
several resistant lines in store for future use in the replacement of
susceptible lines of multiline varieties.
Merits of Multiline Varieties
1. All the lines are almost identical to the recurrent parent in-
agronomic characteristics, quality etc. Therefore, the disadvan-
tages of the pureline mixtures (Chapter 11) are not present in
the multiline varieties.
2. Only one or a few lines of the mixture would become suscep-
tible of the pathogen in any one season. Therefore, the loss to
the cultivator would be relatively low.
3. The susceptible line would constitute only a small proportion
of the plants in the field. Therefore,’ only a small proportion
or the plants would be infected by the pathogen. Consequently,
the disease would spread more slowly than when the entire
population was susceptible. This would reduce the damage tc
the susceptible line as well.
« stner Approaches to Breeding of Self pollinated Crops
Demerits ©f Maltiline Varieties
' 1. , The farmer has to change the seed of multiline varieties every
few years depending upon , the change in the races of the
pathogen.
2. There is a possibility that a sew race may attack all the lines
• of a multiline variety.
Achievements
Multiline variety appears to be a useful approach to control
diseases like rusts where new races are continuously produced. In
India, three multiline varieties have been released in wheat (21 aestU
mm). Kalyan Sona, one of the most popular varieties in the late
sixties, was used as the recurrent parent to produce these varieties.
Variety ‘K3ML 3’ consists of 8 lines having rust resistance genes
from.Robm, Ghanate, K.l, Rend, Gabato, Blue Bird, Tobari etc.
Multiline ‘MIKS 11* is also a mixture of 8 lines ; the resistance is
derived from E 6 254, E 6056 3 E 5868, Frecor, HS 19, E 4894' etc.
The third variety, KM L 7406 has 9 lines deriving rust resistance
from different sources.
POPULATION APPROACH TO BREEDING OF
SELF-POLLINATED CROPS
Self-fertilization of F t hybrids leads to a very rapid increase in
homozygosity. After only 4 generations of self-pollination, about
94 per cent of the genes would become homozygous. Even
in F 3 , half of the genes are in homozygous state. Thus self-
fertilization quickly separates the progeny from a hybrid into a large
number of purelines. As a consequence, selection in such a segre-
gating population only picks out the gene combinations present in
the population primarily as a result of recombination in F 2 . This
reduces the chances of recombination between linked, especially
tighty linked, genes and of recovery of rare transgressive segregates.
There is no opportunity for changing the genotype of the plant
produced by recombination in F is F& and, to some extent, in F».
Thus the two obvious limitations of breeding methods based on self-
pollination of the hybrid (e.g., pedigree and bulk methods) axe: first,
the recombination is limited- to two or, at best, three generations,,
and second , there is no possibility for further changing the genotype
of the segregates.
A population breeding approach has been suggested to over-
come these problems. In population breeding, outstanding F 2 plants
are mated among themselves in pairs or in some other fashion. The
intermating of selected F* plants restores heterozygosity in the ’ pro-
' geny, which provides for a greater opportunity . for recombination.
This also brings together the desirable genes from different F 2 , plants
and would help in the accumulation of favourable genes in the inter-
' mated population. Thus the chances of the recovery of transgressive
VARIETY
Selected parents are hybridized,
YEAR
Fi harvested in bulk
(!) F 2 is space* planted
(!l) Outstanding plants, are selec-
ted and inter mated'
THIRD
V£AR
(s) Progenies from matings are
space-planted
(ii) Outstanding plants are identi-
. fied and intermated
491 ,® 7 ,
■ /EAR
(I) Progenies from matings ar©
space planted
(if) The population is bandied
according to the pedigree
method
#SPTM
year
, COM T ! ISlU E O « 55 L F “POLL NAT 1©N
AMO SELECTION
PURE -LINES ISOLATED
ANO EVALUATED AS IN
THE PEDiGP-Ec ViEThOD
Fig. 16.1. Schematic representation of the population approach to breeding of
self-pollinated crops.
Other Approaches to Breeding of Self pollinated Crops 259
Fs or F 4 progenies would be more desirable* Intermating of selected
•plants may be conti nut ed for two or more generations.
The Idea of population approach was first suggested by Palmer
in 1953. It' is .not Commonly used at present, but may find a greater
application in the future as improvements due to the pedigree method
would become less and less marked. Evidently, the population
approach is akin to recurrent selection commonly used in cross*
:poilma,ted crops (Chapter 1 8), • and often it is referred to as such*
The chief' limitation of recurrent selection in self-pollinated crops is
.■•the difficulty in Making the large number of required crosses by hand
(emasculation and pollination). This difficulty maybe overcome toy
.using genetic ■ or 'cytoplasmic male sterility. When genetic male
‘.sterility "is used, selection is confined to the male sterile (ms ms)
plants in each generation. Seeds from the selected male sterile plants
are generally harvested in bulk. The progeny from such plants inay be
..expected to have both male sterile (ms ms) and male fertile (My ms)
^plants in almost equal ’proportion. Further, the seeds ' produced' ’on
the male sterile plants would be produced by pollination by the male
fertile plants in the population. Thus the use of male sterility
•effectively ensures intermating among the plants of a population and
-eliminates the need for tedius and time consuming hand emasculation
.and pollination.
Results from recurrent selection are available In tobacco and
soybean. In tobacco, Matzinger and coworkers selected the plants
before flowering and intermated them. A linear response of 4.9
.and 7 per cent per cycle to selection for decreased' plant height
and for increased leaf number, respectively, was obtained for five
.•cycles of selection. Further, there was no evidence for a reduction
in variability as a result of the selection. Brim and coworkers carried
out six cycles of recurrent selection for increased protein content in
two segregating populations of soybean, and three cycles of selection
for yield and three cycles of selection for high oil content in another
■segregating population. There was an increase of 0.33 and 0.67 per
*cent/cycle in protein content of the two populations, of 5.3% per
cycle in yield and. of 0.3% per cycle in oil content These findings
amply demonstrate the ■effectiveness of recurrent selection- in j
Improving yield and yield traits in self-pollinated crops.
Iff 1970, Jensen proposed a comprehensive breeding scheme
■which, provides for the three basic functions of a - versatile breeding
programme. Firstly , it allows the development of F 2 , F s etc.- (selfing
series) at every stage of the breeding • programme, which permits
the isolation of purelines for use- as commercial varieties. Secondly ,
it requires intermating among the selected plants/lines in each stage;
the progenies’ from- these intermatingS' form the ■ basis for the
■ next stage of the selfing series in the breeding programme. Thus .
the breeding- programme progresses in two different directions';
(!) Vertically , through the. selfing series leading to the isolation of
-commercial varieties, and' (2) horizontally , through intermating
FIFTH
YEAR
26Q plant Breeding : Principles and Method®
Ear - vUmy -wj-s-g, ax ssarsft
£**d Tbh teSbj dwn is known as Dlallel Selective Matin?
SS(DSM)and i^d J. «J ^““,1 £S
Smiz frw ^s.rr /
S sterS To overcome this difficulty in the same way as m the
rrourrent selection scheme discussed earlier. Further, DSM ,b«J
more complicated than the simple pedigree method which stiJ as the
favourite breeding method for self-pollinated crops.
•.FIRST
YEAR
SECOND
YEAR
PARENT
DIAilFL
SERIES
(?d
["diaLlel j
AMONG I
1 PARENTS]
F, DIALLEL
SERIES
<P.>
-J D! a LLEFi
I AMONG j
fjS i
FIRST
SELECTIVE •
MATING
SERIES
m
SECOND'
SELECTIVE
MATING '
SERIES
(Pd
THIRD
YEAR
MASS
SELECTION
tQ
:r
FOURTH
YEAR
L Ifil
MASS
SELECTION
,
SELECTED
, f 2 plants !
INTER MATED
SIXTH
YEAR
F 6
LINE
SELECTION
MASS
SELECTION
n^
keuLK)
^r
j SELECTED
~4 F, PLANTS i
J INTERMATEDj
~T 1
TT
EVALUATION
A schematic representation of the selective diailel mating scheme.
. The seed produced by Fi plants In each (Pi, P®» Pa, P* etc.) series-
are subjected to mass selection for seed characteristics, and the 1
selected seeds from all the crosses in a series are composited to estab-
lish' a bulk Fa population. The Fa and subsequent generations
may be handled according to bulk, pedigree or single seed descent
schemes. New germplasm may be introduced in any of the Pa, Ps, P*
or subsequent series by mating the new lines with some of the select-
ed plants from the previous series, that normally serve as parents-
for intermatings to generate the concerned series.
261
•Outer Approaches to Breeding of Self-pollinated Crops
Merits of Population Approach
1. The population approach provides for geaxer opportunities for
recombination. This is made possible by restoring heterozy-
gosity through intermating of selected plants. .
.2* This approach helps in the accumulation of desirable genes in
the population. This also is brought about by the intermating
of selected plants from segregating generations.
Demerits of Population Approach
■ L The success of this approach depends upon the identification
of desirable plants in F 2 and the subsequent segregating gene-
rations. This is very difficult, if not impossible, for complex
characters like yield which show low heritabifity. This may be
avoided to some extent by using later generation (F s or F 4 )
progenies replicated yield data may also be used.
2 . Another drawback of this approach is the intermating of
selected plants. This may become a major limitation in some
crops because crossing in many self-pollinated species is
difficult and time consuming.
3. The time taken to develop a new -variety through population
approach would be always greater than that by the pedigree
method.
4. There is no convincing evidence for the benefits from the popu-
lation approach. It has been argued that increased recombi-
nation may be detrimental as it would break the desirable
.linkages. But such a criticism assumes that all or most of the
new gene combinations (recombinations) will be inferior to the
■existing ones. Such an assumption is not entirely valid
since crop improvement is based on the creation of new and
desirable gene combinations.
"SAPID ISOLATION OF HOMOZYGOUS LINES
In self-pollinated crops, the breeders aim at developing superior
'homozygous lines. In crossing programmes, the segregating materials
have to be carried to at least F s or, often, Fg before the' lines become
reasonably homozygous to permit preliminary evaluation. The pro-
gress under selection upto this stage is hampered by- heterozygosity
and it takes 4-5 years to obtain partially homozygous lines. Techni-
ques are now available, at least in some crop species, for the isolation
-of completely homozygous lines from the Fx generation itself. This
•.technique consists of (1) extraction of haploid plants from- F* plants
using anther culture or distant hybridization, and (2) chromosome
•doubling of such haploid plants to obtain completely homozygous
diploid plants, and, subsequently, progenies.
A generalized scheme for rapid isolation of homozygous lines
Is outlined in Fig. 16.3. Selected parents are hybridized (First year) -
262
Plant Breeding : Principles and Methods?
FIRST
YEAR
SECOND
YEAR
0 0 0 3
CO oj
p.
THIRD
YEAR
FOURTH
YEAR
FIFTH* '
EIGHTH
YEAR
NINTH -
YEAR
Fig. 16.3,
j<? 0 O]
lo O O i
F,
[ANTHER culture
’ "OR
INTERSPECIFIC
HYBRIDIZATION
Selected lines are mated
Fj seeds space-planted, preferably
under controlled environment
(i) Anthers from Fi plants cultured or
an appropriate culture medium
(si) Haploid plants regenerated.
(i) Regenerated plants planted in a
green houses
(ii) Chromosome doubling through'
colchicine or some other agent
(Hi) Seeds from individuals plants-
harvested separately
■ (i) Individual plant progenies grown
in held
(ii) Superior progenies harvested
separately
(i) Preliminary yield trial with suitable
checks.
(ii) Outstanding lines included in co-
ordinated yield trials
(i) Multilocation yields trials with
suitable checks..
(il) Outstanding progeny released as &
new variety
Seed increase for distribution to
farmers
Production of homozygous lines through anther culture or inter-
specific hybridization in breeding of self* pollinated crops.
TISSUE CULTURE
LABORATORY
GREEN HOUSE
1
Mi n t in i f
INDIVIDUAL PLANT
PROGENIES
□ f±j □
PRELIMINARY
YIELD TRIAL
fo 0 O 0 o
and the Fj generation is space-planted (second year). It may be desi-
rable to grow the F* under controlled conditions since the environ-
ment is likely to influence the success’ of anther culture from these*
. plants. -Anthers of an appropriate developmental stage from the F a
plants are inoculated on a suitable culture medium (see Chapter 26) in-
order to ultimately obtain haploid plants either through direct
. embryogenesis from pollen grains or through callus and, lately
pfantlet development from pollen grains. The haploid plants are
anally transferred to soil and are subjected to, ordinarily,- colchicine-
treatment in order to double their chromosome number. It is essen-
tial to standardise both anther culture and chromosome doubling
techniques in order to maximise the recovery of haploid and doubled
haploid (homozygous diploid) plants, respectively. Haploids may
•fee -extracted from Fi -plants through a suitable distant hybridi-
zation. In barley, this is achieved by crossing the F x plants* to
Hordeum bulh&sum ; the young embryos from this cross are gene-
rally raised through embryo culture.
Other Approaches to Breeding of Self pollinated Crops
■*%»§
Seeds from individual doubled haploid plants are harvested
separately and are used to plant individual plant progeny rows in
the third year. The individual plant progenies may be planted in an
augmented design (single rows of progenies in an unrepSicated trial
with a suitable check variety recurring every third* fifth* seventh or
eleventh row) to facilitate the elimination of weaker progenies fey com-
paring them with the contiguous control or the moving average. In
t he fourth year 9 superior progenies are planted in a preliminary yield
trial having suitable checks. Outstanding progenies are identified
and evaluated in multilocation yield trials .under the concerned all
India coordinated crop improvement project from fifth to eight years .
The outstanding progeny, if any, may be released as a new variety
and its seeds multiplied and distributed to farmers in the ninth year .
This represents a saving of at least 3-4 years in the develop-
ment of a new variety. But the availability of a well equipped tissue
culture laboratory and dependable green house facilities are a pre*
requisite for this technique to be of practical importance. Off-season
nurseries may be used as a substitute for green-houses* but this would
involve considerable transport and travel efforts* which in most cases
may be prohibitive in. view of the expected gains. For a few crop
species* areas may, exist where two or more crops are taken one after
the other, e.g., rice in parts of Orissa, Tamil Nadu* Kerala etc. Such
special situations may be exploited where they exist* but for most
crops the necessity of green house, facilities is a prerequisite. Another
limitation of the technique is the limited number of crops for which
reliable techniques for haploid production are available ; further
research efforts are likely to enlarge the list of such crops in the
future.
In this method* homozygosity is achieved in a single generation*
while it may often be desirable to maintain heterozygosity for several
generations to permit rare recombinations to occur* The breeder
may combine the technique for rapid isolation of homozygous lines
with a suitable scheme for population, improvement or recurrent
selection to permit a rapid recovery of homozygous lines at the end
of selection period (Fig. 16.4).
HYBRID VARIETIES
Seif- and often cross-pollinated crops show little or no loss in
• vigour or yield due to inbreeding. But F, hybrids in such crops are
generally more vigorous and higher yielding than- either of their
parents. - They are also more stable phenotypically than the parental
purelines. The superiority of an Ft oyer its parents is known as
heterosis or hybrid vigour . Heterosis is commercially utilised by
using Ft hybrids as" commercial varieties, le. 9 hybrid varieties.
Hybrid varieties are much more common in cross-pollinated species
than in self-pollinated species for two obvious reasons. Firstly,
cross- pollinated crops 1 are well suited for the production of large
quantities of hybrid seed since tfaev are naturally cross-pollinated.
Plant Breeding : Principles and Methods
FIRST
YEAR
Selected parents hybridized
F i space-planted ; harvested
in bulk '
SECOND
YEAR
(a) Phenoiypic&Ify superior
plants selected before onset
of flowering
[ii) Selected plants mated in
pairs.
THIRD
YEAR
(i) Seeds from mating® plant-
ed fa progeny rows
(ii) As in (I) and (ii) of the
third year
(0 Seeds from matings planted
in progeny rows
(si) Anthers from superior
plants cultured on a suit-
able medium
(iiO Haploid plants regenerat-
ed from anther cultures
Civ) Haploid plants planted in
green bouse
(v) Chromosome doubling of
haploid plants. Seeds of
individual plants harvested
separately
(!) Individual plant progenies
grown
(ii) Superior progenies select-
ed ; each .progeny har-
vested in bulk
THIRD
YEAR
FOURTH
OR
LATER
YEAR
' ANTHER culture
OR
INTERSPECIFIC
■ HYBRIDIZATION
CHROMOSOME DOU BUNG]
NEXT
YEAR
NEXT
YEAR
Preliminary yield trial. Supe-
rior lines selected
Coordinated yield trials. Out-
standing lines released as
a new variety
NEXT
YEAR
Seed multiplication of the
new variety.
NEXT
YEAR
Fig. 16.4 Production of homozygous lines through anther culture
specific hybridization -after one or more generations of in
of the selected plants In self-pollinated crops.
Other Approaches to Breeding of Self pollinated Crops
As a result* the cost of hybrid seed production is comparatively very
low in cross-pollinated than in self- pollinated species. Secondly * in
■self-pollinated crops superior pureiine varieties give considerably
high yield and they are easily multiplied and maintained. But in
■■■cross-pollinated crops heterozygosity must be maintained to prevent
inbreeding depression. In spite of these, hybrid varieties have been
■used commercially in some seif-pollinated crops like tomato, brinjal,
tobacco, cotton and jowar*
Many F t hybrids in wheat (F aestivum) give 25-30 per cent
higher yields than, the average of the two parents, while in jowar
(S. bicolor ) the increase in yield may be as high as 25-40 per cent*
Outstanding hybrids in jowar may outyield the standard varieties by
a. margin of 50-80 per cent. Hybrid vigour is known in almost all
the self-pollinated crops. But in many crops it has not been possible
to utilize .heterosis commercially because the production of large
• quantities of hybrid seed in 'these crops is very costly and often
. extremely difficult. The hybrid seed is produced by using cytoplas-
mic-genetic male sterility or by hand pollination.
Use of Cytoplasmic-Genetic Male Sterility. Cytoplasmic male
sterility is known in many self-pollinated crops, e.g wheat, cotton
(Gossypium sp.), tobacco (. Nicotiana sp.), sorghum (S..hicohr) 9 chiles'
>{ Capsicum annuum) etc. But in most of the cases, it is not suitable
for hybrid seed production due to one or more of the following
.reasons.
L The male sterile cytoplasm has undesirable side effects, .e.g% s
tobacco {Nicotiana sp.).
2. Effective restorer genes are not available, e.g., cotton {Gassy*
pium sp.).
3. .Natural cross-pollination and seed set in the male sterile line is
not satisfactory, e.v., chillies {Capsicum annuum ).
• 4. The, male sterile cytoplasm may produce incomplete male
sterility.
Cytoplasmic-genetic male sterility is commercially. used in jowar
to produce hybrid seed. A similar system is available in wheat,
but it is not yet commercially exploited. The cytoplasmic male
■ sterility in jowar was discovered in 1950 when the chromosomes of a
. kafir variety were transferred in the cytoplasm of a milo variety, A
male sterile line thus produced, viz., Combine Kafir 60 MS, has been
•commonly used for hybrid seed production in jowar. The fertility is
restored by a dominant gene Msc, which occurs mostly in milo varie-
ties or in varieties of milo origin. The production of hybrid seed '
involves the following steps (Fig. 16.5).
{a) Development of Male Sterile Lines. Hybrid seed production may
utilise the already existing male sterile lines* known A lines* e.g. p
: Combine Kafir 60 MS in jowar. New male sterile lines may . be
developed by backerosslng the mate sterile hybrid from a cross bet-
ween a male sterile line (used m female) tad a male fertile line (used
Plant Breeding : Principles and Methods.
■ MAINTAINING
MALE StERiLE
AND .RESTORER
UNES
CYTOPLASMIC MALI*
STERILE LINE
(A LINE)
221 9 A
SM ISOLATION
MAINTAJNEW
LINE'
(8 LINE)
'22198 '
■. RESTuheR LINE
(R LINE)
CS 3541
SEED FROM THE
FEMALE LINE
HARVESTED
SELF. {
POLL, ’NATION ’’
9 221 3A
f {CYTOPLASMIC
MALE STERILE
LINE
fW ISOLATION
HYBRID SEED *
PRODUCTION
f
r f CS 3541
& ( RESTORER 4.LM0 )/■
SEED FROM .
THE FEMALE
LINE HARVESTED
CSH 6
(MAL£ FERTILE HYBRID)
Fig, 16.5* Scheme for producing hybrid jowar using cytoplasmic- genetic male
sterility.
as male)* .to the male parent.. About 6-8 backcrosses would be
sufficient to obtain all the genes • of the male fertile line in the male
sterile cytoplasm. The new male sterile line* the A line, will be
identical with the male fertile line, known as £ line or znaintainer
line, used in the bankeress programme.
(ft) Mazstesaace of Male Sterile Lfaes." The male, sterile lines (A
lines) are maintained by crossing them with the male fertile lines
(B lines) from which they were developed by backcrossing. The B
lines are known as maintainer lines because they are used for main-
taining the male sterile lines* Thus the male sterile lines are main-
tained by continuously backcrossing them with their male fertile
parent.
(c) Production of Hybrid Seed. For production of hybrid seed, the-
Is ■sterile line is pollinated with the pollen from a restorer line.
The Restorer line carries the gene or genes for restoring male
fertility is the presence of male sterile cytoplasm.' It must combine
well with the male sterile line to produce a high yielding and desir-
able hybrid. In jowar, generally six rows of the male sterile Sine are
planted with two lines of the restorer line. Seeds from the male
sterile line are harvested since they are hybrid in origin.
' a Several outstanding hybrids have been developed in jowar
using cytoplasmic-genetic male sterility* Some of the hybrids released:
267
Other Approaches to Breeding of Self pollinated Crops
Hand Pollination. Production of hybrid seed bv hand pollination is
commercially feasible in crops like tomato {Lycopersicon esculentum),
brinjal [Solarium melongena) and tobacco because each fruit in these
crops produces a large number of seeds. For example, one fruit in
brinjal and tobacco produces about 2,500 seeds. Hybrid tomatoes
produced by hand pollination ,are commercially grown in Europe.
In India, hybrid cotton is produced by hand pollination and is com-
mercially grown at a large scale. The area under hybrid cotton is
currently more than one million hectares. Hybrid cotton is produced
by emasculating the Sowers of one variety ■ and pollinating the
emasculated flowers with pollen grains from the other parent variety.
Emasculation is done one day before the flower would normally
open and the flowers are pollinated immediately after emasculation.
For emasculation, the corolla is cut away with a pair of scissors and
the anthers are removed with a pair of wide-tip forceps. Pollen is
collected in a small piece of soda straw. The soda straw is then
slipped over the stigma of the emasculated flower. The bracts are
pulled up and tied together to hold the soda straw in place* This
technique generally gives about 75 per cent seed set.
Several hybrid cotton varieties have been released for commer-
cial cultivation ; hybrid varieties obtained by crossing two different
strains of G. hirsutum are R 4 (H=hybrid), JKBy 1, Godavari
(NHH 1) 5 Suguna, H 6 and AHH 468 (PKVHy 2), while those
derived -by crossing one strain of G. hirsutum with a strain of"
G. harhadense are Yaralaxmi, CBS 156, Savitri (RHR 253), jaylaxmi
(DCH 32) and K 2HC. H 4 was the first hybrid variety developed by
the Gujarat Agriculture University (Surat station) and -released in
1970, Yaralaxmi was the first interspecific hybrid variety developed
by the University of Agricultural Sciences- (Dharwad station) and
released in 1972. India is the first country to commercially exploit
hybrid vigour in cotton. All the hybrid varieties have been developed
in the tertaploid cottons and no hybrid variety has been released is
the diploid cottons. These hybrids yield 30-40* q/ha seed cotton with'
32-36.5% ginning outturn ; the liber length ranges from 24.5 to
33 mm, i.e. 9 they have extra long fiber. In' addition, a male sterile line'
Gregg is being used for hybrid seed production. AH India Coor-
dinated Cotton Improvement Project is extensively using this mate"
sterile fine to reduce the cost of hybrid seed. Several varieties deve-
loped- in this; manner are in the various stages of testing prior to their"
release for commercial cultivation.
SUMMARY
In self-pllinated crops, multiline varieties may be used to reduce losses
dee to diseases like rusts in wheat. Intermating of selected F a or later genera-
■ tlon plants fronra cross would help in obtaining rare recombinations and in
accumulating favourable genes. Such a population approach to breeding of
selfpollinated crops may become increasingly important in future. In many
self-pollinated crops, ex*, cotton, jo war, tomato,, rice etc,, Fa -hybrids are used-"
is varieties. The hybrid varieties may be produced by hand pollination,
■ jn cotton, tomato, tobacco, etc., or by cytoplasmic-genetic male sterility^#,*.
Plant Breeding : Principles and Methods
in jo war, and rice. Heterosis is common in almost all the self-pollinated crops.
But hybrid varieties are not used in most of them due to a very high cost of
• hybrid seed, and due to a great difficulty in obtaing large quantities of the
hybrid seed.
QUESTIONS
1, ' Differentiate between the following :
(i) multilines and mixed populations, (ii) multiline varieties and varieties
developed through mass selection, (in) population approach and hybrid
varieties, and (iv) restorer and maintainor lines.
2. Define multilines. Discuss their production, uses, merits and demerits.
Describe briefly the achievements through multilines in India.
.3, Briefly describe the contribution of the following scientists : Jenson,
dorlaug and Palmer,
4. What breeding methods would you suggest In the following situations ?
(i) tight linkage between desirable and undesirable traits, (li) a crop
species h attacked by a disease that has many physiological races, (ill) the
character under improvement has low heritability, (iv) the character Is
mainly governed by additive gene action, and (v) the character Is chiefly
governed by nonadditive gene action.
d». Discuss the relative importance of the following breeding methods :
(i) multiline and the population approach, (ii) multiline and hybrid
varieties.
*•6. Discuss the scope of hybrid varieties in self-pollinated crops. Briefly
describe the method of production of hybrid varieties in cotton and jowar.
7. What is the population approach of breeding self-pollinated crops ?
Describe a generalised procedure for population approach to breeding and
discuss its merits and demerits.
Suggested Further Reading
Borlaug, RE. 1959. The use of multibreed or composite varieties to control
airborne epidemic diseses of self-pollinated crop plants. Proc» First Inter-
national Wheat Genetics Symposium, pp. 12-26.
(Brsm, C-A. and Stuber, C.W. J973. Application of genetic male sterility to
recurrent selection schemes in soybeans, Crop Sci. 13 : 528-530.
Frey, K.J. 1981, Plant Breeding II. The Iowa State University Press, Ames.
«-»— - —
^Si'c“VS?"': ! 3«.!50 4 ' M,S! • t,Klion “ d ”*“<« •>*»» to
• CHAPTER 17
Effectiveness of the Different Breeding!
[ethods'for Seif-P oliinsited Crops
The mass and purelic >s selection methods are applied to
objectives, definite require u P tioQto the segregating gene-
fore, it is not su ‘ t ®“ t ®[J 0 TiX F t can be combined with other breeding:
rationsfromc ros ses s a!tho 8 . needs. Of the remaining twe
methods, e.g-, pedigr , * ,p DO ji; n ated crops, the pedigree
f bffSe most $$££ wtile Jbulk procedure
method \r^j # y nf a cariosity for most breeders* m®
? .iSA *->«• b '“ di ’ 15 pro “ d “ re forself '
pollinated crops.
pedigree method ,
At present, ft* pedigree ”f“ ^dofcS* ta *»”' ^
In India, this method appears to be ^“^Ssofthis method is
breeders, of self-pollinated .crops : ySded. How'-
evident from the number ^‘“P^ reflect the relative efficiency of
ever, this fact does not necessari ily reflect « le n a pg may simply
the procedure, The large number o of other methods,
be due to its extensive use, almost to he & may be doe
. Further, the overwhelming For example, (i) it is the
to reasons other than its effect . „ egreS ating generations
earliest devised method for the h^f^f^lfbrleders for the
from crosses. (2) »Pr^.£gSS^ W bS«l»8 ffom ^
selection of sopenor and desirable OTes 8 g r eeders get the
earliest segregating generation, that is, r* v /
.270
Plant Breeding': Principles and Methods
satisfaction of applying their ‘skill from F, onward, i.e., they are
active participants in the breeding programme. In addition,
(4} the method is so doubt highly effective in changing qualitative
characters and highly heritable quantitative characters in the direc-
tion of selection. However, the crucial question is, ‘is selection
during the segregating generations.for quantitative characters with
moderate to low’heritability effective ?’
Conventionally, visual selection is the basis of PS, but doubts
have been expressed about its efficacy (Shebeski, 1967). Some
authors concluded that a visual selection for superior yielding lines
was ineffective as there was a poor agreement between visual ratings
and actual yields of lines (see Boerma and Cooper, 1975). Shebeski
(1967) advocated the use of contiguous control for early genera-
tion yield testing (early testing) of individual plant progenies.
It has been reported that an analysis of covariance is likely to
increase the effectiveness of contiguous control technique in the
identification of superior yielding progenies. But the reports on the
usefulness of early testing are confiicting. Some workers have
reported that early testing is able to identify higher yielding lines,
while others found it to be ineffective (see, Salmon et a!., 1978 ;
Boerma and Cooper, 1975 ; Knott and Kumar, 1975).
Estimates of correlation between yields of different generations
have been used as a measure of the effectiveness of visual and early
testing selections. There is a considerable variation among the
correlation estimates reported from different studies. In general,
significant correlations between yields of Fs lines and those of later
generation lines derived from them have been observed but in some
studies the correlation estimates were nonsignificant. Further, it is
doubtrut that the significant correlations (even if / = 0.60). justify
the effort and expenditure involved in early testing (see, Knott
and Kumar, 1975).
In case of wheat, McGinnis and Shebeski (1968) reported that
the identification of high yielding F a plants by these selectors
was successful, but the correlations between F 2 plant yields and F„
plot yields in the. following season were not significant. In other
similar studies, visual selection among Fg lines resulted in an yield
increase, but this increase was relatively smaller as compared to that
from selection based on plot yields (see, Salmon et al. 1978). Salmon
et . , < 1 9 7 8 > developed low yielding, high yielding, random and
visual Fg bulks in four crosses of tritic&lc*- These bulks were .made
up of Fj lines selected for low yield, for high yield, randomly and
visually respectively, and were evaluated at two locations. The
yields of visual and high yielding bulks were comparable in the four
crosses at both the locations. In most of the cases, the random
bulk, although numerically inferior, was also comparable to the
visual and high yielding bulks (Table 17.1). .Clearly, early testing in
F s was not superior to visual selection (pedigree) in the identification
fSItii
Effectiveness of the Different Breeding Methods 271
fable 17 1. Mean yields of F, bulks derived from four crosses of triticale
at two locations (Glenlea-Carman ; combined) in Manitoba*
Canada and one location (Ciano) m Mexico
(based on Salmon et al* 1978)
Mean yield { gjplot )
Selection group bulks
Ciam
G leniea- Carman
83 X Kmla~3
Low yield bulk**
Random selection bulk
Visual selection balk
fiigb yield balk**
Low yield bulk
Random selection bulk 1ST Jab 51
Visual bulk ■ 237. 2b 631,5b
High yield bulk . 233,1b S68.Tb
(6 TA 204 x Bronco 90) X ( Ab-terX BrmadUh)
Low yield bulk 129 338.5a
Random selection bulk 1^.9ab 437.0o,
Visual bulk ’ ' 188 * 9 ^ ^.<te
High yield bulk ISMb 49 /.2b
KoaIa-3x6iTA 513
Low yield bulk 270.0a
Random selection bulk 258 Ja
Visual bulk 271.2a 651.3b
High yield bulk 279.2a 640.,b
* Values followed by the same letter are not’ significantly different, compari-
sons within crosses within locations® ^
Low yield and high yield bulks developed on the basis of yield data on -Ft
lines.
•of superior yielding lines of triticale. However, early testing effectively
identified poor yielding F» lines.
It emerges from the foregoing that visual selection in early
segregating generations of crosses appears to foe as. effective in the
identification of high yielding lines/plots as actual yield tests (early
testing). This raises an important question of cost and benefit on
the usefulness of early testing, Another major disadvantage of
, early testing is that it restricts the number of progenies that can be
evaluated. Further, there is a risk in early testing of rejecting low
Yielding lines which may generate desirable recombinants in the
subsequent generations ; in fact, this has been suggested as one of
the serious weakness for PS as well (see later).
Plant Breeding : Principles and Methods
SINGLE SEED DESCENT
The idea of single seed descent (SSD) was originally suggested,
by Gouldan in 1939 and subsequently modified by Brim in 1966.
Brim referred to this procedure as modified pedigree method, and
this name has often been used in the literature (see, Snape and
Riggs 1975). However, the procedure lacks both the essential
features of PS, viz., maintenance of pedigree records, and individual
plant selection during the segregating generations. In all respects,
it is essentially a variation of the bulk procedures aad should be
regarded as such (Tee and Qualset, 1975). The method has aroused
considerable interest among plant breeders and offers an unique
opportunity for rapidly advancing segregating generations. The
merit of the method is evident from both theoretical consideration.'
and experimental findings, as well as from its ever increasing popula-
rity among the breeders.
Theoretical Considerations. Theoretically, the mean expression of
a character in F : and its distribution in F s will depend on genetic
architecture of the character, but the distribution of F„ lines obtained
through SSD will be more or less the same irrespective of the
genetic situation (Snape and Riggs, 1975). In the absence of domi-
nance, the F t and F« genotypic distributions will be similar, but
more of the F 6 lines will be as good as the extreme homozygote,
that is, the homozygote containing all the positive alleles for the
trait. But when a large amount of directional dominance is
exhibited, the F 6 distribution will fall markedly on the lower side of
F 2 distribution, and only about 2% of the F 6 lines will exceed the
F 2 mean. Similar results are expected in the case of complementary
epistasis. in the presence of duplicate epistasis, the Fg -distribution
will be markedly skewed to the right (higher expression of the trait)
and about 40% of the individuals will have the phenotype of the
extreme homozygote. The distribution of F 8 lines will be much
less skewed and will be similar to that in the absence of dominance,
but only about 6% of the F« lines will be comparable to the extreme
homozygote. Further, the genetic advance expected from PS and
SSD will be comparable if there is no dominance or in' the presence
of duplicate epistasis, but in the presence of marked directional
dominance or complementary epistasis the former will be superior
to the iatter. ' /
Cockerham and Matzinger (1985) evaluated the effects of the
nuniber of generations of selfing on selection response in SSD
populations. They used a quantitative genetic model for an arbi-
trary number of alleles and loci with additive, dominance and
additive x additive gene actions. The mean relative response to
selection increased markedly with the number of generations (0.68,
0.83, 0.92, 0.96 and 0.98 for 1, 2, 3, 4 and 5 generations, respec-
tively) of selfing. Thus selection in a SSD population may be
initiated in F 5 or Fe ; little further gain is expected by delaying it
further. The type of gene action would have some effect on the;
Effectiveness of the Different Breeding Methods 273
selection response but this effect would be greatly reduced with the
increased number of generations of selling.
In a breeding programme, however, the breeder is more
concerned with the proportion of homozygous lines that are superior
to the better parent involved in the cross than with the mean and
variance of all the possible lines that can be extracted from the
cross. Jinks and Pooni (1976) developed a simple procedure for
estimating the probability of obtaining inbred lines that fall outside
the parental range or exceed the Fi, if it shows heterosis, by a
specified amount. In the case of simple additive, dominance and
additive environmental model, the probability of an inbred line
having a higher score than P x (the superior parent of the cross) or
a lower score than Pi (the inferior parent) will be approximated by
the two-tail normal probability integral corresponding with the
value given by the following formula provided the distribution of
inbreds derived from the cross is approximately normal.
id)
V (D+E)
where, (d) is the additive gene effect, D is the additive genetic
variance and E is the additive environmental variance. In practice,
E may be ignored since a breeder would be interested in the average
performance of inbreds over environments.
In case the Fj shows heterosis, the. probability of obtaining-
inbreds superior to ■ the Fi will be approximated bv the one-tail
normal probability integral corresponding with the value obtained
from the following formula.
Jh) _
ViD+E)
where ( k ) is the dominance gene effect.
In the presence of additive x additive epistasis (i), the estimates
of probabilities become biased. If i is positive, the probability of a
derived inbred exceeding the superior parent Pj will be less than
that of a derived inbred being inferior to P 2 ; the reverse is the case
when* is negative. If the distribution of inbreds is normal, the
probability of obtaining inbreds that score higher, than Pj will be
the one-tail normal probability integral corresponding with the value
yielded by the following formula.
id) + (i)_
V D+I
Similarly, the probability for inbreds scoring lower than P a
will be the one-tail probability integral corresponding to the value
obtained from the following formula.
— id) -f- (/)
VTd+iT
Pl ani Breeding : Principles and Methods
274
where, i is the additive X additive epist^is gene
additive X additive genetic variance. earlier generations of
to obtain a precise 5 estimate :o ^ fee biaged
crosses, but m such cases, tb of the above two probabi-
upward due to a significan • ^ outside the parents!
SS-d-J if •> allowance for
significant i is not made.
These probability «*** byJ^A
theorobaSty^hile a predominantly coupling phase linkage would
lend o ove esdmate it. The presence of linkage and its predomi-
tend tc loveres c ted bv comparing the variances of F 2 and
fJ/'SSSS. ftSSv Spto test cross analysis The presence
nf oenotvDe x environment interaction is not expected to present a
S long as the interaction is a linear expansion or contrac-
a “ te ?f,?he phenotypic differences between genotypes are
magnified in one environment relative to the other with little or no
change in ranking. In practice, this seems to be the most likely
form the integrations would take unless the environments are
particularly diverse (Jinks and Pooni, 1976).
Subsequently, Pooni and Jinks (1978) extended the method for
oredicting the probability of an inbred exceeding the Pi with respect
to two three or more characters simultaneously. The parameters
reauired for these predictions can be obtained from the six basic
generations, viz.. Pi, P& Fi> Pi> 62. aiK * aD( ^ f; tn P^ e *® st cross -
The oredicted proportions of inbreds exceeding the parental range
for a single character (Jinks and Pooni, 1976) as well as those for
two or three characters considered together (Poom and Jinks, 197a)
showed a good agreement with the values obtained from the inbreds
derived through SSD from simple intervarietal crosses in Nicotiana
rustica. Further, there was a close agreement between the predicted
and observed proportions even when the distribution of the inbreds
derived from the crosses was significantly nonnormal (Pooni et ah,
1977).
Pooni and Jinks (1979) considered the various sources of bias
in the parameters used for the prediction on the properties of the
inbreds produced from a cross by SSD. They used the data from
N. rustica crosses for testing the validity of their predictions. They
concluded that the estimate of D could introduce appreciable bias
in the predictions if epistatic gene effects were significant. In the
absence of epistasis, estimates of D from F 2 and the two backcross
(Bi and Bg) generations were adequate. But in the presence of
epistasis, D estimates obtained from Fs families appeard to be the
best, while those from F s and backcrosses were the worst.
Experimental Results. Homozygous lines derived through SSD
generally exceed the parental limits for various traits, indicating that
transgress*''!? segregants are retained during the procedure. For
Effectiveness of the Different Breeding Methods 275
example, in safflower 64 of the 379 BC 2 F 4 lines derived through SSD
from the cross (SH 202 x Oleic Lead) XSH 202 oufykldffi S
higher yielding parent SH 202, 95 lines had higher oil content than
Oleic Leed (the superior parent for the trait), and 62 exceeded the
superior parent m terms of yield X oil content (Fernandez-Martinez
and Dominquez- Jimenez, 1981). ruDez
a ^nrJint r L ,S <j| e n era K ly u° me p ~ m 3oss when a Population is bandied
according to SSD, which may affect gene and genotype frequencies in
such populations. Martin ef al. (1978) investigated ^effects of
population density on gene frequencies for some qualitative and
f char f ters m three cr °sses of soybean handled accord-
!r a %n S/nLiT genera f tl0ns - At l he iow Population density
(a. 170 plants/m ), gene frequencies for qualitative characters
(flower colour and pubescence colour) did not deviate from the
expected ones, while at the high population density (Ca. 510 plants/
m-) there was a significant deviation in most of the cases This
significant deviation was attributed to the high plant loss at the
higher population density. The means for certain quantitative
characters were also affected by population densitv : progenies from
the high density SoD population tended to be slightly later in
maturity (m3 crosses), were slightly taller (in two crosses), had
slightly higher lodging score (in two crosses), and had slightly
bigger seeds (in one cross) than those from the SSD populations
grown at low density. Generally, these differences in the population
means were relatively small in magnitude. However, the among
line variances for the four quantitative traits did not differ signifi-
cantly between low and high density SSD populations of the Lee
crosses, except for seed size in one cross where the variance from the
low density population was significantly higher.
Plant Loss. In each generation, some seeds would fail to geiminate
n^ts P mav S f^fti d fi d,e dUC !? insect and disease attacks, while some’
plan s may fail to flower and set seed. Consequently, a proportion
of plants may be lost in each generation and the population size
tf^LFr° Sr r SS! | Ve y t!ecl,ae WItil the advancing generation due to the
nh^ifn t^ ( ° n ° De S£ f- d f f° m u ach survivin S Plant composited to
obtain the next generation) set by the SSD procedure. Results from
7 < d *‘ ca in i 1181 SSD P°P ula tion size declines at rates
ranging from 4.75 to 30.3 per cent of the population size in each.
fnne a at T Tafci J? 17-2 ;!‘- The ma S nilude of plant loss in these studies
Of the exnerimL d t CP M d , 1Dg l, UPOn the Cr ° P Species and the conditions
liclfh! P t- M ng . bean a PP ears to be more prone to plant
!o Stac herC a° P speciess probably due to its high susceptibility
to _ diseases and environmental stresses. Similarly populations
mamtamed at a higher population density may suffer a greater plaS?
1978) han th ° Se Cu tlvated at Iower Plant densities (Martin et al,
. ^bus plant loss appears to be a serious weakness of SSD
scheme. In situations of relativpfv hiah n t an t j
Table 17.2. 'Plant loss in the segregating po pulations of some crops handled according to SSD scheme.
Population density Plant loss . Generations advanced in Referenct
(plant si m‘) (per centlgenerauon )
276
Plant Breeding : Principles and Methods*
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Effectiveness of the Different. Breeding Methods
genotypes retained in an SS0 population 'may not be a random'
sample from the population. At least two approaches have been'
suggested fox' preventing plant loss. Firstly , 2-4 seeds from each
plant of the previous generation may be planted in a single hill and
thinned to one .plant/hill at the time of flowering. This -scheme
involves considerable labour in harvesting, maintaining and planting
the 'seeds of each plant separately and.. later in careful ■thinning.
■"Further, re some situations it may fail to achieve the desired result
-(Singh eial, in press).
' Alternatively, two seeds from each plant or all the seeds from
■one pod of each plant (Byfh et al P 1979) may be bulked to raise the
next generation. This modification amount^ to a restricted bulking
minus' the effects of natural selection. In the early, generations,
^populations obtained through such * scheme may not be markedly
-different in their -composition from those derived -through 'the
, conventional SSD scheme. However, as the homozygosity increases,
■such populations may become closer to bulk than SSD populations.
."Such, a modification of the SSD’ scheme may, therefore, fail to
prevent the shifts in - gene and genotype frequencies, due to random
'drift from plant loss and due to sampling error, .although it may be
:abfe to maintain the population size.
The plant loss, therefore, may lead to an • erosion in genetic
variability of SSD populations. .However, the • results reported so '
far (Martin et a!., 197 S ; Singh et ah , in press), do not support this
•■expectation.
PEDIGREE Vs. SSD
Casali and Tigchelaar (1975a, t>) compared PS, SSD and BP
(bulk procedure) through computer simulation studies. They found
that for characters with high heritabilitv, PS was the most effective,
while SSD was the most, effective for traits with low fieri labilities.
Farther, SSD was effective even for traits having very low heritabi-
Tity (10%).. SSD -also offered the greatest benefits in situations
where simultaneous selection for several characters having different
heritabilities has to be done (this essentially is the situation in
.almost all breeding programmes).
. A comparison between the effectiveness ' of PS and SSD
methods is afforded by the results from several studies. In . general,
the lines derived through SSD are comparable, .in some -cases
•superior, to those developed through PS. Knott and Kumar (1975)
■selected 300 plants from among 2,500 F 2 plants of each of the two -
wheat) crosses, viz., Wisconsin 261 xPitic 62 and Wisconsin 261 X
■ Maottou. -The F 2 plants were selected for resistance to rusts and
root rot, straw strength, height, head type, maturity and seed type.
The same selected F 2 plants were handled according to SSD as well
as PS. The SSD procedure was followed in F*, Fa and F 4 to obtain
1F 5 lines ; some' F s lines with obvious weaknesses were eliminated
278
Plant Breeding : Principles and Methods
and the remaining were yield-tested in F 6 . The 300 F 3 lines from
each of the two crosses were also grown in separate replicated yield
trials ; the P 4 progenies were derived from the selected F 3 lines (on.
the basis of yield test) and advanced in an off-season nursery to F fi .
The Fs SSD and Fg PS lines from a cross were planted in a replica-
ted yield trial ; separate ' trials were planted for each cross, The
mean yields of PS lines were significantly higher than those of the
SSD lines in both the crosses mainly because there were fewer very
poor yielding PS lines (Table 17.3). Correlations between the F 8 and
Fg yields of PS lines were significant in both the crosses but they
were relatively low (0.29 and 0.14). A comparison between the
yields of F s PS lines derived from the highest yielding 20% of F 3 .
lines and the highest yielding 20% of the SSD lines revealed that in .
each cross the SSD lines were at least as good as the PS lines^
(Table 17.3).
Bcerma and Cooper (1975) extracted Fs lines from four
soybean crosses according to SSD or PS, the latter method was
* either based on visual selection or on early testing. The SSD lines-
were yield-tested in F 7 and a variable number of the highest yielding
lines were included in the Fg yield tests. Mean yields of all the
selected lines, the mean yields of the five highest yielding lines, and
the yield of the highest yielding line from each population showed
mo consistent differences between the procedures of selection (SSD
and PS). The lines from early testing procedure were consistently
later in maturity than those from SSD and visual selection. The-
authors concluded that SSD was the most efficient since it required
less selection effort, avoided expensive yield testing until later gene-
rations, and allowed a more rapid generation advance.
links et ah (1977) compared the performance of a random-
sample of 59 F? lines developed through SSD from a cross*
(V2x : V1 2) of N. rustica with that of lines selected according to PS -
for high and low mean performance in combination with high and
low sensitivity to a macroenvironment variable (planting dates) for
two morphological traits ({lowering date and final plant height).
Evaluations of these lines were made in eight different environments.
In all the environments, few to several SSD lines were better than
the PS lines (Table 17.4). These results demonstrate the superiority of
SSD over PS for the isolation of superior recombinant lines. The
inefficiency of PS was attributed to seasonal differences. Further, it:
was concluded that mean performance and environmental sensitivity
were largely under the control of different genes.
The effectiveness of early testing in F 3 and SSD procedures in.
two cowpea (Vigna unguiculata) crosses was compared by Ntare
et ai. (1984). Both early testing and SSD populations were initiated
from the same selected F 2 plants in each cross. The F s SSD and
early testing lines were compared in yield tests. There was signi-
ficant correlation (r=0.51 to 0.85) between the yields of Fs parental
amd F@ progeny lines. Further, there was significant correlation
Effectiveness of the Different Breeding Methods
2SQ Plant Breeding : Principles and Methods
T.M» 17 A The distribution of 59 SSD lines derived from the cross Vi x V,. of
Tabie 7. rustica f 0 r flowering time and final height in conjunction with
hi e h and low sensitivity to environment, and the number ot bbD
lines superior to the two selected lines for each charcter environ-
mental sensitivity combination (based on Jinks et at lv/ /).
Mean
performance
of the character
Environmental
sensitivity
Number of
SSD lines
Number of SSD lines
superior to the two
appropriate pedigree
selection lines
Flowering time
High
High
II
1
High
Low
12
12
Low
High
20
20
Low
Low
16
J
Final plant height
High
High
21
5
Is
High
Low
9
y
Low
High
13
13
Low
Low
16
1
between visual ratings of F s and F« lines for yielding ^llity n
their actual yields indicating that it is possible to visually menu y
high yielding cowpea lines. However, the gram yields of obU a
early testing lines were comparable, indicating ■ a lack of superiority
of the early testing procedure. Similarly, there was no difference
between SSD and PS lines -of N. rustica with respect to seven quan-
titative characters. These lines were extracted from a heterotic
cross. Further, the data on these lines confirmed that inbred Sines
which outperform the heterotic Fi can be readily extracted from it
by either SSD or PS method (Jinks and Pooni, 1984).
Dahiya et aL (1986) compared the performance of mungbean
(V. radiata) lines extracted from a broad base population (developed
by ordering 15 different purelines in complex crosses). These lines
were developed through pedigree (visual selection for, two genera-
tions), SSD and early testing (each for two generations). bbD
appeared to be the most effective as more SSD lines ©etyielded the
best check. The mean yields of all SSD lines, !G% top-yielding SSD
lines and the highest yielding SSD line were the highest. The cany
‘testing procedure was found to be the least effective. -Although the
differences in mean yields of SSD and PS lines were consistent,
their magnitude was rather small.
These data do not indicate any marked advantage due to the
pedigree method (with or without early testing) over SSD. This is
mainly because the selection for yield and several other quantitative
traits is relatively ineffective in the early segregating generations.
281
Effectiveness of , the Different Breeding Methods
Therefore, ' SSD procedure seems preferable to PS since the latter
involves selection effort, maintenance of pedigree records, expensive,
> yield trials for early testing, and imposes a limit on the lines that
can be handled during the segregating generations. In the SSD
procedure, on the other hand, there is little, if any, selection effort
and yield test expenditure, no limitation on • population size, no evi-
dence for any loss in genetic variability, and an opportunity for a
very rapid advance of generations (opto 4-5 generations/year with a
good green-house facility). Further, a selection for highly heritable
agronomic characters may be practised in F s while initiating the SSD
population from a cross (Knott and Kumar, 1975).' However, such
a selection may result in the rejection of high-yielding lines before
they are recognised as such. This may partially explain the relative
ineffectiveness of PS. The lack of selection in SSD, except perhaps
in Fa, allows the production of character combinations through
recombination which might not have been selected for in' PS ; ■ such
recombinants may not be of high agronomic value but may never-
theless be high yielding and of potential value as breeding stocks
(Snape and Riggs, 1975),
DOUBLED HAPLOID TECHNIQUE
The doubled haploid (DH) technique of obtaining homozygous
lines of self-fertilized crops is becoming increasingly popular, parti-
cularly in rice, wheat and barley, as it represents a saving of 4-5 years.
Theoretically, the means and variances for a quantitative trait will be
similar for the populations of SSD and DH lines irrespective of the
.genetic architecture of the trait, provided there- is (1) no linkage and
(2) no selection of gametes 'or genotypes in either population (Snape,
1976). In addition, the frequency of each homozygous genotype is
•expected to be 0*, where k is the number of segregating loci.
But the means and variances of the SSD and DH populations
would -differ in the presence of linkage ; the direction and the magni-
tude of these differences would depend on the initial linkage phase,
the recombination frequency and' the - presence of epistasis. In the
■cases of coupling phase linkage with complementary epistasis or
repulsion phase linkage with duplicate epistasis, the mean of DH
;populatioo will be higher than that of SSD population, .while the
reverse will be the case when coupling phase linkage is associated
with duplicate epistasis or repulsion phase linkage is present with
complementary epistasis* Variances* of SSD populations will be
.greater than those of DH populations if the positive alleles are
initially present in the repulsion phase, but the opposite will be true
if they are initially in the coupling phase. Epistasis will tend to
* increase this difference in the case of repulsion phase, but it will
reduce the difference when alleles are present in coupling phase. -Since
in most plant breeding situations repulsion 'phase linkage may be
expected to predominate, SSD may be expected to be superior to the
‘DH technique in releasing the maximum variation from a cross. This
. is mainly because recombination between the linked genes is limited
282 Plant Breeding : Principles and Methods
to F a in the case of DH, while in SSD it continues to occur till later
generations, albeit in progressively decreasing frequencies. The
superiority of SSD procedure may be expected to be the maximum
at intermediate recombination frequencies (p=0.293 for coupling,.
and p=0.17 for repulsion p== recombination frequency ; Snape,
. 1976).
DH lines of tobacco developed through anther culture followed
by midrib culture of haploid plants are genetically stable and do not
show detectable ^cytogenetic anomalies (see Schnell et aL 1980).
Similar results have been reported for barley . DH lines derived
through interspecific hybridization with Hordeum bulbosum (see,
Park et al , 1976). DH lines developed from plants heterozygous for
qualitative traits showed typical Mendelian segregation ratios in
some studies, while in other reports evidence for a preferential deve-
lopment of some gametes into plants were obtained. In addition,
DH populations derived through anther culture from highly inbred
parents show variability for quantitative traits as well as reduced
vigour. This variation has been attributed to : (1) residual hetero-
zygosity in inbred parents, (2) mutagenic effects of colchiciog (used
for chromosome doubling) and (3) mutagenicity of the anther culture
technique itself (see, Schnell, et al , 1980). However, studies with
cotton and barley indicate that there may be no disadvantage
associated with complete homozygosity of DH lines, and that their
yield stability is not affected (see Park et al , 1976).
Schnell et al (1980) obtained 50 DH lines through anther cul-
ture and 50 random Fe lines through SSD,, from a single F ± plant
from the cross Hicks Broad Leaf X Coker 139 of N. iabacum.
The performance of DH and SSD lines, alongwith suitable checks,
was evaluated in three environments. The DH population yielded,
on an average, 12.6% less cured leaf, had inferior leaf quality, was
later in dowering, produced reduced leaf numbers and its cured leaf
was lower in total alkaloids than the SSD population. The DH
.population showed a greater genetic variation for leaf yields that the
SSD- population, but the lower mean in the case of former negated
the benefit of increased variation. For ■ example, none of the DH
lines was even equal to Coker 139, the best check, for leaf yield,
while one SSD line outyielded this check. Although the predicted
gain in leaf yield from selection in the DH population (7.8%) was
larger than that: in SSD (4.5%), the selected SSD lines were expected
to be markedly superior (2,436 kg leaf/ha) to the selected DH lines
(2,246 kg leaf/ ha). It was, therefore, concluded that selection in SSD
populations would be preferable to that in DH population for the
identification of superior genotypes. The possible causes for the
inferiority of the DH population were suggested to be (1) incomplete
inbreeding in the SSD population, (2) repulsion phase linkage,
(3/ mutagenic effects of colchicine treatment, (4) mutagenicity of the
anther culture procedure, and (5) differential development of micro-
spore genotypes into embryoids.
Effectiveness of the Different Breeding Methods
Park et ah (1976) developed 100 lines through each of the
three procedures, viz., SSD (Fe), pedigree (visual selection, F 6 ) and
DB (haploids obtained through interspecific hybridization with-
Hordeum bulbosum), from two barley (ff. vulgare) crosses. . From
these lines, 52 random lines for each procedure in each cross were-
included in yield trials at two locations over a period of two years.
There was no difference for grain yield, heading date and plant
height between the DH lines and those derived through SSD and PS
in terms of means, variances and the frequencies of desirable geno-
types (Table 17.5). Further,- there was no evidence for deleterious-
effect of complete homozygosity in the DH lines ; the DH lines were
as good agronomically as the SSD and PS lines. It was concluded
that the DH technique is a very useful tool for producing high yield-
ing homozygous barley lines. ' “
Choo et a/. (1982) developed DH (through the H. bulbosum
technique) and oSD lines from two barley crosses. The frequency
distributions of DH and SSD lines for grain yield, heading date and
plant height were compared by Mann- Whitney U test, Kolmogorov-
Smirnov two- sample test and Wald- Wolfowitz runs test. The distri-
butions of DH and SSD lines for the three metric traits were com-
parable. Although the SSD procedure allows a greater opportunity
for recombination than the DH method, it did not produce a sample
of recombinants which differed significantly from those derived
through the DH technique.
The performance of barley lines developed through PS and DH
(anther culture) techniques was compared by Feidt and Forougbi-
Wehr (1986). Five PS and 5 DH lines were extracted from each of
the five barley crosses ; they were planted in a replicated yield trial
in different environments (represented by locations and years). The
performance of PS and DH lines was comparable in three of the five
crosses. In one of the remaining two crosses, two DH lines were
significantly lower yielding than PS lines, while in the other cross-
two DH lines were significantly higher yielding than PS lines.
Clearly, the DH lines represent a nonrandom sample from the five
barley crosses, indicating that , there is a considerable unconscious
selection in the desirable direction during their isolation through
anther culture.
In cereals like wheat and barley, two techniques are available
for the production of DH lines ; (!) anther culture and (2) inter-
specific hybridization with H. bulbosum. The relative merit of these
techniques has been examined in some detail by Snape et al. (1986).
They have proposed three criteria for the comparison of DH pro-
duction techniques : (1) easy and consistent production of large
numbers of doubled haploids of all the genotypes in the breeding
programme, (2) production of normal and genetically stable DH
lines, and (3) absence of selection during the production of DH lines.
Both anther culture and H. bulbosum techniques are quite successful
with some genotypes of barley and wheat, but not with others.
:284
Plant Breeding : Principles and Methods
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Effectiveness of the Different Breeding Methods 285
However, a steady progress is -being made in improving the efficiency
of these' techniques. At present, the H. bulbosum technique appears-
to be markedly more efficient in producing green haploid plants than
the 'anther culture technique both in wheat and, particularly, in
barley (Table 17.6). Both the systems require comparable . facilities
•and financial commitments. But in terms of man-hours required for
the production of every DH line, anther culture (0.3 man-hours/DH
.line)- appears to be relatively more efficient than the H. bulbosum
technique (0.5 m§n«hours/DH line).
Table 17.6. The relative effectiveness of anther culture and H. bulbosum
techniques in generating haploid lines of wheat and barley (after
Snap e era/., 1986)
Method ' Anthers cultured
or
florets pollinat-
ed
Albino plants
or
seeds set *
Green plants
Green plants
per 100 anthers
or per 100
florets ** '
Anther culture
28,000
Wheat
890
3.2
H, bulbosum system
1,0>0
—
55
3.4
-Anther culture
7,020
Barley
54
10-
0.14'
H. bulbosum system
1,908
1654
218
11.40
*Albino plants in the case of anther culture, and seeds set in the case of
H, bulbosum system.
**Pet 100 anthers in the case of anther culture system, and per 100 florets in.
the case of H . bulbosum system.
-There is a considerable evidence to show that both the techni-
ques induce genetic variation in addition to that generated by segre-
gation, and recombination in the F x hybrids; In- addition, these
techniques appear to impose a selection pressure so that the lines
derived through them' are essentially nonrandom. Snape el al (1986)
feel that critical studies are required to assess the usefulness of such
variations and of selection pressures in breeding programmes. A
■comparison of spring barley DH lines derived from two crosses
through the anther culture and the H. bulbosum techniques showed
that the latter had significantly superior yield/plant (in both the
crosses) and' 100-grain weight (in one cross) than the former, while*
both the groups ■ had comparable grain number/spike. However,
more data are required from other crosses in barley and from those
in wheat to allow a meaningful generalisation.
Thus, DH lines are either comparable or inferior to SSD. lines
in yield and other quantitative traits, but in some cases, they may be
comparable to PS lines. The development of DH lines either through.
riant Breeding : Principles and Methods
anther culture or through Interspecific hybridization (for embryo
rescue) requires dependable tissue culture and greenhouse facilities.
On the other hand, a rapid isolation of SSD lines requires only a
dependable greenhouse facility. It is doubtful that the saving of
one year through DH procedure over SSD (assuming 3-4 generations
oer year with the help of a greenhouse) would justify the effort and
■expenditure involved in the production of DH lines (particularly in
the Indian context)* In most developing countries, however, only
few plant breeders would have an access to tissue culture and green™
house facilities. Often, these facilities may not be wholly depend-
able leading to frequent losses of valuable breeding materials. Conse-
quently, breeders of developing countries would generally have to
depend on off-season nurseries for advancing the generation of their
materials ; in such a situation, SSD appears to be the only feasible
.procedure for a rapid isolation of homozygous lines.
BULK METHOD
When breeders handle segregating generations from crosses of
self-pollinated crops according to the bulk scheme, they have some
•explicit or implicit expectations which may be summarised as follows.
I n long-term hulks y natural selection is expected to (1) improve the
mean performance of bulk populations, and (2) to retain agronomi-
cal^ superior genotypes in the population, or even (3) cause an
increase in their frequency thus facilitating their isolation in later
generations. (4) In short-term bulks , on the other hand, it is hoped
that there would be no shift in gene and genotype frequencies due to
either natural selection or random drift. Thus in the case of long-
term bulks, the gene and genotype frequencies of the population are
expected to change for the better, while short-term bulk populations
are hoped to contain a random sample from the all possible homo-
zygous genotypes obtainable from the initial population.
The effectiveness of natural selection has been .studied in
varietal mixtures and in both long and short-term bulks. In addi-
tion, many workers have combined mass selection with the bulk
orocedure fRP) to achieve specific goa 1 *.
varietal Matures. Data on changes in frequencies of different pure
lines are available from studies on varietal mixtures of several crops,
e.g., barley (K vulgare ), wheat (T. aestivum ), soybean (G. max),, rice
(O. sativa), rajma (P. vulgaris) etc/ In general,' the frequencies of
different varieties constituting a mixture undergo marked changes
within few generations, and one or two genotypes tend to become
predominant with time. Further, different varieties may become
predominant at different locations or under different environments
(see Allard, 1960). The ability of a genotype to survive in mixture
is generally referred to as competitive ability or fitness .
Evidently, different varieties of various crops differ in compe-
titive ability. But the crucial question is whether competitive
Effectiveness of the Different Breeding Methods 287
ability is positively associated with yielding ability and/or agronomic
desirability. Theoretically, ,f yield and fitness are positively assSS
ted, superior yielding genotypes would tend to became predominant
Consequently BP would be more efficient than PS and SSD S
dures. farther, genotypes having high seed set and lower seed
weight (or seed size) may be expected to be mere competitive San
bolder seeded genotypes ; this appears to be the case in many crons
(e.g.. Khalifa and Qualset, 1974, in wheat : Hamblin 197 s sn
lentils ; Roy, 1976, in wheat). However, smaller-seeded ’genotypes
■even if higher yielding than bold-seeded ones, may not be des rfble
from the commercial viewpoint. °i e
In some early studies, agronomically poor barley varieties were
found to be poor competitors in varietal mixtures; but in Zmt
other investigations the competitive ability was not associated with
yielding ability (see Allard, 1960). Suneson (1949) concluded that
the bulk popuktion method of breeding will not necessarily penS
tuate the highest yielding or the most disease resistant progenies but
that otherwise intangible character of competitive abifity^ay me£
sure other very important characteristics*. These apprehensions have
since been supported by data from several studies. In wheat (Khalifa
and Qualset, 1974), rice (Jennings and Dejesus, Jr. 1968) and
soybean (Mumaw and Weber, 1957) survival in varietal mixture*
was not related to yielding ability. In wheat, rice and bariev natural
selection tends to favour tall, leafy and high tillering pl'ant types
while the high yielding ideotype is short with narrow! erect leaves
and fewer tillers. Results obtained from studies on the segregating
generations of crosses in wheat (Khalifa and Qualset, 1975) and riel
(Jennings and Herrera 1968) support these .conclusions. Similarly
s ri m fj 1 ; se t de . d ’j? w y iel <*wg rajma (P.vulgaris) genotypes tend toelimS
nate high yielding, large-seeded types (see Hamblin and Evans 1976)
In addition, determinate (bush) type soybean and rajma genotypes
Table 17.7 Performance of DH lines derived from two barley crosses tfirou*
anther culture and H. bulbosum techniques (after Snape et at..
Method Number of Grains'!
lines spike
100-grain
wight (g)
Yieldjplant
(g)
Anther culture
if. bulbosum
Mid-parent value
Anther culture
B\ bulbosum
Mid-parent value
E 1388x1506 C
19
25.02
5.203
11.73
19
. 26.34
5.668*
V 13.95*
—
26.59
5.333
12.84
21
Franken X Trumpf
25.38
4.775
11.06
21
26.30
4,875
13.08*
.25.88
4,291
11.14
l«fvedDH d fnel nCebCtWe<!nthemeanS ° f aather «d‘ure an dH. bulbosum
^ ,, * 4
2gg n««» - - •*
oo a ; n st indeterminate (vine) types
bold irue far o.he,
and Muehlbauer, 1981).
natural selection tends to act against cert
— they appear to have poor s
Obviously, it would favour only those cl
reason
(Haddad
Clearly
features of some crops^ since
in mixtures. C , _v:“' , r.
that are positively associatec
Long-Term Bulks. Long-Ter crosscs aic u , i ,, ul <., U v U -
rations) of segregating g * , v : P w; n « renotvpes would increase,
the hope that frequencies of with time due
while those of ^^*fflSta«been conducted on long-
t° natural select.on Several studie. h^e ^ j%2 . chQO £{ ^
IWO Mac S Harvdy^ 1982h Most of these , studies have been
1980 » Maw ana ftar y, Q f barley . a composite cross may
: <«) bdlkjl ! eq-
genejarcu m uu generations of surveral crosses,
(^involving “e I pZ^sIlLs i § nto a complex .ross and 13)
( 2 ) involving sever p eroge neous male sterile line aiongwith
«- s " d f ™” -*
sterile plants only (Mac and Harvey, 198.).
In most of these investigations, it was found that the mean
perfortn “of bulk popohuL — - n™-,d «
reported by Suoeson and Stevens m 19 53 (see Aimra, tyou;
remarkable in that the yield of Composite Cross J 1 (CC II) of barley
increased from 67.6% of Atlas m F 3 -F 4 to 135./% oj the same
variety in F 21 (Table 17.8). This represents an increase of about 68 /
in thewield of CC II over a period of 20 years of bulking, which
amounts to an average yield increase of 3.4/ per generation. This rate
of yield increase compares favourably with that obtained through
mass selection in cross- pollinated crops (see Chapter 8). These find-
ings become more remarkable when one considers the performance of
Table 17 8 The effect of hoiking period on the yield of Composite Cross II
(CC II) of barley (After Suneson and Stevens, 19 d3)
Yield of CC U
as per cent of
Atlas
Yield (Bushels' acre)
Year of
evaluation
Generation of
Composite
Cross 11
Effectiveness of the Different Breeding Methods 289
pure-lines isolated from the different generations of CC II. The top
three selections from F gc of CC 1 1 outyielded Atlas by an average
of 56%-in four years of replicated trials ; these selections were also
moderately good in agronomic characteristics. Similar encouraging
results were obtained from some other long-term barley bulks (see
Allard and Jain, 1962 ; Jain 1961).
Ia contrast, results from other studies have not been as
dramatic Allard and Jain (1962) and Jain (1961) examined in detail
the materials developed by Suneson and concluded that the yield
and fitness of barley Composite Cross V increased upto the
eighteenth generation, the latest generation evaluated by them. Simi-
lar results have been obtained with the populations of several self-
pollinated species which had been grown as bulks for many genera-
tions (see Allard, ei ai„ 1968). Most of these evaluations were,
carried out at low density, which would have promoted profuse
tillering. However, at commercial crop densities genotypes would
not produce many tillers despite their high tillering capacity. In
addition, high tillering capacity may not necessarily reflect high
yielding potential at high crop densities. Some authors even fee!
that when vield per unit area is the objective, high tillering capacity
inav be a' nuisance. it has been clearly demonstrated that
data obtained at low densities are of little use in determining the
vield potential of crosses at crop densities. In addition, the compe-
litive ability of different genotypes may change with population
density (see 'Hamblin and Evans, 1976).
Choo et a!. (1980) grew 17 single cross bulks of bailey at
two locations ; comparisons were made by growing Fio and F, 6
generations of each of the 17 populations at the two locations for
?wo years Thev observed that natural selection favoured longer
awns shorter strikes, smaller flag leaves, and fewer spikes per plot,
but it had little effect on the number of grains/spike or gram yield.
The effect of location was noticeable only in F, 5 op awn length,
spike length, Sag leaf area and number of grains/spike.
The exploitable genetic variation present in Composite Cross
XXI (CC XXI) of barley was estimated by Mac and Harvey
(1982). CC XXI was generated by compositing seeds from
6,200 entries from USD A world collection and 7,500 male
<?ter ; ]e nlants having Coast; Manchuria, Trebi, Hanchen, (
CC XIV and CC XV backgrounds ; seeds from the male sterile
plants were harvested in bulk in 1961 and distributed as CC XXL
Mac and Harvev isolated 284 lines from generation
(G4) and 307 lines from G12 of CC XXL The mean performance
of G 12 lines showed an increase in heading date, maturity date and
grain yield, but a decline in kernel weight as compared to G 4 lines.
However, the coefficient of variation for these characters was com-
parable in the two populations. They concluded that although com-
posite crosses show slow progress, they are inexpensive to .mamtam
290
Plant Breeding : Principles and Methods
and that they can provide useful reservoirs of variation which would
complement the traditional world collection.
Short-Term Bulks. Many plant breeders do not feel enthusiastic about
maintaining long-term bulks simply because of the long period of
time involved. Still BP may oe utilized for the isolation of homozygous
lines with the least expenditure and effort ; for this purpose, short-
terra bulks of 5-8 generations are maintained. It is, however, neces-
sary to examine if changes in gene -and genotype ■ frequencies take
place in short-term bulks. Many studies have focussed on this
question, and several authors have attempted to compare the effec-
tiveness of short-term BP with SSD and PS.
Khalifa and Qualset (1975) grew F s to F 8 generations of two
wheat crosses as bulks under field conditions, and recorded the
height of individual plants. Yield as well as mean height of the
population increased, while the frequency of short-statured plants
decreased with generation. Mean yields of lines extracted from F s
F 4 , F 5 and F 6 generations showed a significant increase with genera-
tion. Some of the F 7 lines (derived from F« bulk) were superior
to both the parents in yielding ability. Evidently, there was a
directional and stabilizing selection for both plant height and yield
in these short-term bulks.
The effect of short-term (upto F e ) bulking in two high-yielding,
and two low yielding crosses of rajma (P. Vulgaris) was studied by
Hamblin (1977). Bulking did not alter the mean performance of the
two high yielding crosses, but there was a steady increase with gene-
ration in that of the two low yielding crosses. In this study, the
progeny variance of crosses did not markedly differ from the parental
variance possibly due to a large environmental (interplant competi-
tion) effect on the expression of quantitative traits of this crop. It
was concluded that “cross variance, for characters which are much
influenced in their relative expression by the environment (of which
yield is the outstanding example), has little relevance as a criterion
for deciding the relative worth of different crosses at crop densities’.
These studies clearly demonstrate that shifts in gene and geno-
type frequencies take place even in short-term bulks, and that in
most cases the performance of bulks improves with generation. How-
ever, sometimes natural selection may act against a desirable geno-
type, e.g., short-statured plants in wheat etc. In such cases, a mass
selection procedure could be effectively used to eliminate the undesir-
able types from the population without much labour.
Mass Selection in Balk Populations. Taller plant stature, smaller
seed size and later maturity appear to be the disturbing traits that
are favoured in many bulk populations. The breeder may subject a
bulk population to mass selection for shorter plants, bolder seeds
and early maturity by mowing off taller heads, sieving off
smaller ;eeds and harvesting at the desired maturity time, respective-
ly. The effectiveness of such mass selections has been investigated
by several workers.
Effectiveness of the Different Breeding Methods
29!
Romero and Frey (1966) subjected a heterogenous and segre-
gating bulk oat population to mass selection for uniform plant
height -All the panicles taller than a specified- height were clipped-
off, and at maturity only the top 10 cm of the clipped plots was
harvested. This procedure of mass selection was effective in reduc-
ing mean plant height and the genetic variance for the trait of the
.population. ' In addition, the selected population became earlier in
maturity and gave higher grain yield than the original population.
Fehr and Weber (1968) applied three cycles of mass selection
for seed size and specific gravity in the F 6 -F 8 generations of two
soybean crosses. There was an approximately linear change in the
direction of selection in seed size and specific gravity with selection
cycle ; there was an associated change in protein and oil contents of
the seeds as well. The maximum progress for high protein and low
oil was obtained by selecting for large seed 'size and high specific
gravity. On the other hand, a selection for small seed and low
specific gravity resulted in the maximum progress for ' high oil and
low protein. Smith and Weber (1968) -were able to select for protein
and oil 'contents in soybean by mass selecting for specific gravity
-alone.. Selection - for high specific gravity resulted in a higher -mean
protein and lower oil content, while the reverse was the consequ-
ence of a selection for low specific gravity. The effect of continued
mass selection for specific gravity varied among and within popula-
tions.
Peterson etal (1986) subjected 52 bulk populations of wheat
to mass selection for seed density of water-imbibed (at 0-3°C for
9-10 days) seeds ; the selection was aimed at improving seed protein
content. Selection for low seed density was able to improve seed
protein content in 10 of the 52 populations ; the increase in protein
ranged from 6-il g protein/kg seed. However, seed size was un-
affected by selection for seed density. A large amount of nongene-
tic variation in seed protein content appeared to limit the effective-
ness of selection for seed density.
These results indicate that mass selection for highly heritable
traits can be effectively practised in bulk populations. Mass selec-
tion techniques may also be designed to eliminate highly competitive
but agronomically poor types from bulk populations. The pheno-
menon of correlated response to selection may also be exploited,
■whenever such opportunities exist, to select for a relatively difficult
or time-taking-to-estimate trait indirectly by mass selecting for a
.highly correlated more- simply estimated trait, e.g. 9 selection for
protein and oil contents in soybean through mass selection for seed
^ize and seed density.
BULK Vs. PS AND SSD
Several workers have compared the effects of short-term bulk*
ing with. SSD or pedigree methods on the segregating generations
292
Plant Breeding : Principles and Methods ;
from crosses of self-pollinated crops. The main objective of such,
studies is usually to determine if the inexpensive procedure of
bulking compares favourably • with ihe more demanding SSD, or
with PS, the most demanding of the three.
Muehlbauer et ah (l 981) used computer simulation to compare,
the genetic variation retained after four generations of inbreeding in
BP and SSD populations of a crop with seven chromosomes, each
chromosome having six loci. In P 6 , the additive genetic variance in
the BP population was smaller than that in the SSD possibly due to
the losses in genetic variability during generation advance according'
to BP. Within the bulk population a considerable proportion of the'
total additive genetic variance originated from within line variance as-
opposed to that which originated frorn between line yariance. This-
difference between within-line and betweea-Iine variances was consi-
derable when the inbreeding coefficient was greater than 0.9. In the*
SSD population, on the other hand, ail the. additive variances ^ must,
originate from between line variance. When the standard deviation
of fecundity was greater than 25 seeds/plant, progeny from 75%
of the original Fs plants were eliminated after four generations
of BP. About the same number of lines were lost after four genera-
tions of SSD when plant survival was lower than ^70% in each
generation of SSD. Linkage had little effect on additive genetic
variance in either system, unless the two parents of the original
cross had considerably different sets of alleles.
The effectiveness of BP and PS methods has been compared by
many workers, and often conflicting results have been reported. In
most studies, e.g., those with, barley and soybean, PS and BP we re-
found to have comparable effectiveness. But in some studies, PS
was reported to be more effective than BP in the isolation of high
yielding lines (see Boerma and Cooper, 1975). The findings of some
of the mo*e recent studies are summarised below.
Empig and Fehr (1971) developed four populations, viz SSD,.
restricted cross bulk, maturity group bulk and cross bulk, from F 2
of three soybean crosses and carried them to F 5 according to the .
respective procedures. In 45 random progenies derived from
each of the four populations of the three crosses were evaluated at
two locations. Method means did not differ significantly at either
location, and differences in genotypic variance among methods
were not consistent in all the crosses. SSD, restricted cross bulk and
maturity group bulk maintained a similar number of high yielding
lines, which was about twice as many as cross bulk. SSD and
maturity group bulk were the most ‘ effective in maintaining early
lines. SSD was about twice as effective in maintaining large-seeded
lines as other methods, while restricted cross bulk had the highest
frequency of tall plants. Maturity group bulk was the most time-
consuming at each harvest, followed by restricted cross bulk, SSD,,
and cross bulk in that order. They concluded that SSD was the
least influenced by natural selection ; therefore, it may be the most
useful method in greenhouse or winter nursery environments where
Effectiveness of the Different Breeding Methods 2 93
a genotype may perform differently than under field conditions In
•its area o£ adaptation’*.
Tee and Qualset (1975) subjected two wheat hybrid popula-
tions (F 2 onward) to a rapid generation advance according to BF
and SSD under low nutrient supply in a greenhouse. Random SSD
and BF-derived Hoes 'were evaluated in F 4 , Fs and F® at two.
locations. Generation means showed increased plant 'height and
■yield from Fa to F® in one hybrid but no change in the other hybrid
■with BP. But the means did not change with SSD, except for
•plant height in the cross where recessive genes for dwarfness were
•segregating. Genetic variation among lines was greater for SSD
than BP in one cross, while the other cross showed the opposite
•picture. Mass selection for increased and decreased plant height
■during the accelerated generation advance was effective ; it was
■more effective for decreased plant height. Selection response for
earliness, however, was only slight. Thus the SSD and BP popula-
tions were generally comparable except Tor the competition effect
of plant height, e g. 9 tall plants increased in BP. It was concluded
that “in a system of rapid-generation turnover where only a few
seeds per plant are produced, the BP method can be applied more
efficiently than SSD \ However, when the competition effects are
important, SSD is preferable to BP.
Haddad and Muehlbauer (1981) advanced three lentil crosses
from F a -F 4 according to SSD and BP. In F 4 , 50 random plants
were selected from each population, their seeds were increased in
F 6 and a replicated yield trial was conducted in F 6 . SSD maintained
more genetic variation than BP in 15 of the 21 comparisons of
characters, e.g. 9 for days to maturity and yield, in cross 1 ; height
■of the lowest pod in cross 2 etc. SSD-derived populations had
.about 11% more erect lines than BP populations, while BP
maintained a marginally higher percentage of taller types than SSD.
'This indicated a reduced frequency of short plants with low fiowers
as a result of natural selection operating within ' BP against less
■competitive short types. The authors concluded that SSD is an
efficient cost- saving method of advancing' lentil populations..
Rahman and Bahl (I9S5) advanced six hybrid populations of
chickpea from F 2 to F 4 according to SSD, bulking with mass
selection (MS) and random bulk (BP). In all populations,^ except
■■one, means for seed yield and yield component traits were higher in
MS than in SSD or BP ; the last two were more or less comparable
with each other. The estimates of genetic variances appeared
to follow a similar trend. It was concluded 'that MS was superior
to SSD and BP.
.Pa war et ah (1986) handled F 2 and F s generations of two
wheat crosses according to BP, SSD and PS ; the performance of
these populations was compared in yield tests in Fa-arid F 4 . The
mean performance of F 3 and F 4 PS populations was superior for
grain yield/plant and other yield traits than that of BP and SSD'
populations. Significant phenotypic correlations between the
294
Plant Breeding : Principles and Methods
performances of F* parents and their F 3 progenies, and between the
performances of F 3 parents and their F 4 progenies were observed
for almost all the characters in both the crosses. The SSD procedure
appeared to be superior to BF.
Segregating generates (F 2 -F 4 ) from five mungbean (Vigna
radiata) crosses were handled according to' BP and SSD; SSD popula-
tions were maintained at three different population densities. Two
crops per year were grown in kfaarif and spring seasons. The SSD and
BP populations from each cross were evaluated in replicated trials
in F 3 , F 4 and F 5 generations for yield and several yield traits. Mean -
performance of the BP and SSD populations did not differ signi-
ficantly, except in a few cases, which were most likely due to
sampling/experimental errors. Estimates of coefficient of variation*
heritability and genetic advance were variable, but no consistent
differences between the methods (BP and SSD) could be detected
(Singh, 1987 ; Singh et al , in press). A peculiar feature of this
study was the large environmental effect on the expression of yield
and yield traits of the various populations ; this is comparable to
the findings of Hamblin (1977) with rajma,
CONCLUSION
Any breeding scheme for handling the segregating generations,
from .crosses in self-pollinated crops should ensure the following :
. (1) an accelerated generation cycle, (2) an absence of intergenotypic
competition, (3) a low mortality during generation advance,
(4) maintenance of a large population size, (5) use of environ men ts
where natural selection is minimised, (6) a low labour input require-
ment (Tee and Qualset, 1975) and (7) -a low land requirement (if
off season nurseries/greenhouses are to be used). Experimental
findings reveal that PS is generally not- superior to SSD ; in some-
cases it may even be inferior to SSD. Early testing may marginally
improve the effectiveness of 'PS over SSD, but it is doubtful if the
gains justify the labour and financial commitments in early testing.
Similarly, DH (doubfed-haploid) technique may represent some gain
over SSD in terms of time, but again the labour and fund require-
ments are far too high for DH than those for SSD.
In short-term bulks, natural ' selection tends to favour taller*
smaller-seeded and often later-maturing types in several crops.
But when segregation for these and other characters associated with
fitness is not taking place, short-term bulk populations are
comparable to those derived through SSD. Even where segregation,
for such traits is taking place, the bulk population may be effectively
subjected to mass selection in favour of the agronomically desirable
but less competitive traits. Alternatively, separate bulks, e.g., tall
and short-statured bulks in cereals, large and small-seeded bulks in
Phaseolus etc may be created for such traits to avoid the effects of
natural selection . Bulks may also be subjected to rapid generation '
advance in a greenhouse in , the same ■ manner as SSD provided an
environment that minimises the effects of natural selection is
295
Effectiveness of the Different Breeding Methods
available (Tee and Qualset. 1975), Thus in a noncompetitive situa-
tion, BP is preferable to SSD since the latter involves the harvesting
of a single seed from each surviving plant of the population, and there
is always some plant loss under SSD. However, when intergeno-
typic competition, is likely to occur, SSD is preferable to BP. Thus
SSD appears to be^ the most dependable, most rapid and the least
expensive method for obtaining homozygous lines from crosses of
self-pollinated crops ; in addition, SSD is as effective a breeding
method as PS or often even early testing (PS with yield testing).
These conclusions, are, however, not supported by the relative
success particularly in India, of the various breeding methods (PS,.
BP, SSD, DH) in developing high yielding varieties of various
crops. In this respect, PS is by far the most successful ; in fact,'
not a single variety developed through ’BP, SSD or DH has been
released in India. But the singular success of PS in varietal
development and its extreme popularity with the plant breeders do
not necessarily demonstrate its superiority over the other breeding
methods. In fact, its extreme popularity may be the reason for its
singular success in this country and vice versa . Few breeders in
India use a procedure other than PS for handling the segregating
generations from crosses of self-pollinated crops, and this may
account for the lack of success of other breeding methods in the
development of varieties.
Based on the experimental findings discussed in the foregoing
sections, a rational breeding scheme involving the least commitment
of labour, land, funds and time, and at least as effective' as the
pedigree- method may be developed as follows. It is assumed that
a dependable greenhouse facility is not available, but the breeder
has an. access to off-season nursery facilities*
1. Selected parents are crossed according to an appropriate
mating design- during the main crop season.
2 . The F/s so obtained are grown in an off season nursery to
advance the generation.
3, In the following main season' (second year), the F 2 populations
are space-planted to facilitate visual evaluation of individual
plants. Plants are selected on the basis of simply-inherited
and highly heritable characters, e.g. f disease resistance, plant,
height, lodging resistance etc. Only those traits that are'
governed by dominant genes are' selected for in F 2 ; selection
-for recessive traits is delayed ■ till the F 4 generation. This is
done to avoid the elimination of genotypes that may yield
desirable recombinants in the subsequent generations. Seeds
from all the selected plants from an * F 2 population are
composited. If desired, mass selection for seed size etc. may
also be practised.
4, If a large number of crosses are being handled, F a bulks frbm^
the crosses may be planted in a replicated yield trial (in
296
Plant Breeding : Principles and Methods
addition to the space-planted population; item *3). Poor
yielding crosses may be rejected so that a greater attention
may be provided to" the more promising crosses.
5, The P a bulks (crosswise) from selected Fi plants are grown in
an off-season nursery to advance the generation. Each F s
population is handled according to the bulk scheme.
6. In the main season of die third year, F 4 populations are space-
planted. From each population,, a large number of plants
are selected for simply inherited, highly heritable traits
irrespective of whether they are governed by dominant or
recessive genes. Seeds from all the selected plants of a
population are composited.
7* The bulk F 5 populations (crosswise) are grown in an off-season
nursery to advance the generation.
g. In the fourth year (main crop season), Fe populations are
space-planted, and a large number of plants having desirable
characteristics are selected from each cross. Seeds from each
selected plant are kept separately.-
9. Id the main season of the fifth year, F 7 progeny , rows from
the selected F e plants are grown in an unreplicated trial,
appropriate checks are planted at regular intervals. The
performance of progeny rows is compared according to
either contiguous control or moving average procedure. If
'enough seed is available and the number of F? progenies is
manageably small a replicated trial may be conducted.
Superior progenies are harvested as bulks for further evaluation
in a replicated station trial.
10. In the sixth year, a replicated station trial is conducted ;
outstanding lines (Fs) are identified for multilocation trials in
the next year.
The above scheme (1) permits the breeder to exercise his skill
through selections in Fs, F* and Fo generations, (2) utilizes the
facility of off-season nursery to save at least two years in compa-
rison to FS (seven years for multilocation trials as against 9 years
in the case of FS) and takes only three more years than DH
(multilocation trials in the fourth year), (3) makes the maximum
use of available resources (land, labour, funds and time), (4) avoids
the record keeping of PS, and (5) permits the breeder to handle a
larger number of crosses than would be possible with FS,
In case a dependable greenhouse (GH) facility is available,
selection- may 'be delayed til! F 3 (Fi and F* grown in GH). P 4 and
Fs atf© grown in GH according to SSD or BP, and onward is
handled as described earlier. This represents a saving of one year
as compared to the earlier scheme, and it takes only two more
years than DH, the quickest procedure for obtaining homozygous
lines from crosses.
.Effectiveness of the Different Breeding Methods 297
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Mumaw, C.R.E. and Weber, C R. 1957. Competition and natural selection in
soybean varietal composites. Agron. J. 49 : 154-160.
Ntare, B.R , Aren’ova, Me, Redden, R J. and Singh, B.B. 1984, The effective-
ness of early generation (F a ) yield testing and the single seed descent pro-
cedures in two cowpea (Vtgna unguiculota (L.) Walp.) cresses. Euphytica
33 : 539-547.
Park, S.J., Walsh. K.J., Reinbergs, F., Song. L.S P. and Kasha, K.J. 1976*.
Field performance of doubled haploid barley lines in comparison with
lines developed by the pedigree and single seed descent methods. Can/ J.
Plant Sci. 56 : 467474.
Pawar, I.S., Paroda, R.S. and Singh, S. 1986. A comparison of pedigree selec-
tion, single seed descent and bulk methods in two wheat {: Triticum aestivum
L. em. Thell.) crosses. Crop Imp. 13 : 34-37.
Peterson, C.J., Liu, G.T., Mattern, P.J., Johnson, V.A. and Kuhr, S.L. 1986.
Mass selection for increased protein concentration of wheat based on seed'
dmsiiy. Crop. Sci. 26 : 523-527.
Pooni, H.S., Jinks, J.L. and Cornish, M.A. 1977. The causes and conseQuences -
or non-normality in predicting the properties of recombinant inbred lines-
Heredity 38 : 329-338.
Effectiveness of the Different Breeding Methods 299
PooNr, H.S. and Jinks, J.L. 1978. Predicting the properties of recombinant
inbred lines derived by single seed descent for two or more characters
simultaneously. Heredity 40: 349*361.
Pqoni, H.S. and Jinks, J.L. 1979. Sources and biases of the predictors of the
properties of recombinant inbreds produced by single seed descent. Here-
dity 42 : 41-48.
Rahman, M.A. and Bahl, P.N. 1985. .Comparison of single seed descent, mass
' selection and random bulk methods in chickpea. Indian J. Genet. 45 :
186-193.
Romero, G.E. and Frey, K.J. 1966. Mass selection for plant height in oat
populations. Crop Sci. 6 : 283-287.
Roy, N.N. 1976. Intergenotypic plant competition in wheat under single seed"
descent breeding. Euphytica 25 : 219-223.
Salmon, D.F., Barter, E.N. and Gustafson, J.P. 1978. A comparison of early
generation (F s ) yield testing and pedigree selection methods in triticale.
Crop Sci. 18 : 673-676.
Schnell, R.J..II* Wernsman, E.A. and Burk, L.G. 1980. Efficiency of single •
seed descent vs. anther-derived dihaploid breeding methods in tobacco.
Crop Sci. 20: 619-622.
Shebeski, L H. 1967. Wheat and wheat breeding* pp. 252-272. In R F. Neilson
(ed.) Proc. Can. Cent. Wheat Symp. Modern Press, Saskatoon, Canada.
Singh; R.P. 1987. Comparison of single seed descent and bulk methods In
mungbean ( Vigna radiata (L.) Wilczek). Ph.D. thesis, Ranaras Hindu
University, Varanasi.
Singh, R.P., Singh, B.D. and Singh, LS. Plant loss in SSD populations of
mungbean {Vigna radiata (L.) Wilczek) Indian J. Genet, (in press).
Singh, R.P.* Singh, B.D., Singh, R.N. and Singh, LS. Comparison of bulk
and single seed descent methods in two mungbean crosses. Crop Imp.
(in press).
Smith, R-.R. and Weber, CR. 1968. Mass selection by specific gravity for -
protein and oil In soybean populations. Crop Sci. 8 :• 373-377,
Shape, J.W. 1976. A -theoretical comparison of diploidised haploid and single -
se'ed descent populations, -Heredity 36 : 275-277.
Shape, ’J.W. and Riggs, T. J 1975. Genetics! 'consequences of single seed'
descent In the breeding of self-pollinated crops. Heredity 35 : 211-219.
■Shape, I.W., Simpson, E., Parker, B.B., Freidt, B. and Foroughi-Wehr, B.
19S6'. Criteria for the selection and use of doubled haploid systems in.
cereal breeding programmes/ In Genetic Manipulations in Plant Breeding
(Eds., Horn, Jensen, Odenback and Schieder), pp., 217-229. ■ Walter de-
Gruyter & Co., Berlin, New. York.
Suneson, CA, 1949. Survival of four barley varieties in mixture. Agron. J.
41 : 459-461.
Tee, T.S., and Qualset, C.O. 1975, Bulk populations in wheat breeding r* 1
Comparison of single, seed descent and random bulk methods. Euphytieai
24 : 393-405.
CHAPTER 18
Population Improvement
Populations of cross-pollinated crop species ^ are highly
heterozygous as well as heterogeneous. Their genetic make-up is
such that they show variable inbreeding depression, which in some
cases may be very severe. Consequently, breeding methods for
cross-poilinated crops aim at preventing inbreeding, or at least to
keep it to the minimum, in order to avoid its undesirable effects.
The breeding methods commonly used in cross-pollinated crops may
be grouped into two broad categories : (1) population improvement
and (2) hybrid and synthetic varieties. In the case of population
improvement , mass selection or its modifications are used to increase
the frequency of desirable alleles , thus improving the characteristics of
the population. In the case of hybrid and synthetic varieties, a variable
number of strains are crossed to produce a hybrid population ; the
strains that are crossed are selected on the basis of their combining
ability.
Population improvement was the earliest breeding method
applied to cross-pollinated crops. Mass selection has been used by
farmers since early days and may be expected to have played a
significant role in the improvement of cross-pollinated crops. The
interest in population improvement methods declined with the
development of hybrid and synthetic varieties. However, breeders
are now paying increasingly more attention to population inprove-
ment programmes since the last two decades. The maize improve-
ment programmes at CIMMYT, Mexico and the pear! millet
improvement programme at XCRISAT. Hyderabad, are largely based
on population improvement.. The All India Coordinated Maize
Improvement Project is placing an increasingly greater emphasis
on population improvement. The population improvement methods
may be grouped into two general classes.: (1) without progeny
testing and (2) with progeny testing.
'Without Progeny. Testing. Plants are selected on the basis of their
phenotype, and no progeny test is carried out, e.g., mass selection.
With Progeny Testing. The plants are initially selected on the basis
of their phenotype, but the final selection of plants that, contribute
to the .next generation is based on progeny test. . This class of
.population improvement includes : progeny selection or ear-tb-row
.method, and recurrent selection.
Population Improvement 30 1
A more comprehensive and commonly .used classification, of
selection schemes is presented in Table 18.1. However, the discussion,
in this chapter shall follow .a less detailed, but equally useful and
much simpler classification.
OBJECTIVES OF SELECTION
in self-pollinated crops, selection is employed to isolate plants
with superior genotype's ; these plants are then used 'to establish
separate purelines or their seeds are bulked to produce a mixture of
purelines. This is possible because self-pollinated crops are naturally'
homozygous and generally do not show appreciable inbreeding,
depression. In contrast, cross-pollinated crops generally show
moderate to severe inbreeding depression. Consequently, inbreeding
must be avoided or kept to a minimum in cross-pollinated species..
Farther, individual plants from such crops are highly heterozygous ;
the progeny from such plants would be heterogeneous and usually
different from the • parent plant due to segregation and recombina-
tion. Therefore, desirable genes can be seldom fixed through selec-
tion in cross-pollinated populations, except for qualitative trails-
and,' perhaps, for easily observable quantitative characters with high
heritability. The breeder, therefore aims at increasing the frequency
of desirable alleles in the populations. This would result in an
increase m the frequency of desirable gene combinations or geno-
types. As .a result, the phenotype of the population would bfe
favourably changed. It should be clearly understood that in cross-
pollinated species the genotype of the individual plants is generally of
little importance , particularly in population improvement programmes.
It ds the frequency of desirable genes or alleles in the population as cr
whole that determines the value of a population •
MASS SELECTION
Mass selection is the oldest breeding scheme available for
cross-pollinated crops. In mass • selection , a number of plants are
selected on the basis of their phenotype , and the open-pollinated seed'
from them is bulked~together to raise the next generation. The selected-
plants are allowed to opes-pollinate, i.e., to mate at random includ-
ing some degree of seJfing (usually about 10% in maize, 2. mays).
Thus mass selection is based on the maternal parent only, and 'there'
is no control on the pollen parent. Selection of plants is based on
their phenotype and no progeny test is conducted. Mass selection,
as applied to cross-pollinated crops, is essentially the same as that-
applied to self- pollinated crops. The method is outlined in Fig.
18.1. The selection cycle may be repeated one or more times to
increase the frequency of favourable alleles ; such a selection scheme-
is generally known as phenotypic recurrent selection . Care should lie
taken to select a .sufficiently -large number of plants in order to keep
inhreedinor tn a minimum. The efficiency of mass selection primarily- 1 *
Plant Breeding : Principles and Methods
first year
(FIRST
SELECTION
CYCLE)
OOOOOOO
OOOOCOO
OOOOOOO
ooooooo
ORIGINAL
POPULATION
(i) Several plants selected on the basis of
* ’ phenotype , . •
(ii) Open pollinated seed from the selected
plants harvested and bulked
©ECONO YEAR
(SECOND
SELECTION
CYCLE)
ooooooo
ooooooo
ooooooo
o o o o o o o
SELECTION
CYCLE MAY 8£
repeated one
or MORE TIMES
(j) Bulk seed from the selected plants
grown
(ii) Mass selection may be repeated, /.<?.»
items (i) and (ii) of the first year
repeated
YIELD trials
Fig. 18. 1. Mass selection as applied to cross-pollinated crop species. When the
** ‘ * selection is repeated one or more times, as outlined here, the scheme
is known as phenotypic recurrent selection.
depends upon the number of genes controlling the character, gene
frequencies and, more importantly, heritability.
Merits of Mass Selection
J. Mass selection is an extremely simple breeding progamme.
Work of the breeder is kept to a minimum since the selections
are tased on phenotypes of plants.
2. The selection cycle is very short, i.e., of only one generation.
Thus in every generation, one cycle of selection is completed.
3. It is highly efficient in improving characters that are easily
identified visually and have high heritability, e.g,, plant height,
size of ear, date of maturity etc.
A. If proper care ts taken, mass selection is effective in improving
yields in cross-pollinated crops. Most cross-pollinated crops
have a high additive component of genetic variance whicl
responds to selection.
population Improvement
303
5. Since the improved strain is likely to be similar to the original
population in the range of adaptation, extensive yield trials may
not be required before release as a new variety.
Demerits of Mass Select I oe
1. Selection of plants is based on the phenotype of individual plants*
Most of the quantitative characters are considerably affected by
the environment. Therefore, superior phenotype is often a poor
basis for the identification of superior genotype.
2. The s elected plants are pollinated by both superior and infe-
rior plants present in the population as the selected plants are
allowed to open-pollinate. This reduces the effectiveness oi'
selection.
3. High intensities of selection reduce population size and, as a
result, lead to some inbreeding. Inbreeding depression may
nullify the advances made under selection.
Table 18.-1. A brief description of the various selection schemes for pocula-
lion improvement (the description is based primarily on maize).
Selection scheme
INTRAPOPULATION
IMPROVEMENT
A. Mass selection
1. For one sex
2. For both sexes
3. Stratified
4. * Contiguous control
B. Family selection
1. Half-sib
a. Ear-torow
b. Modified ear-to-row
Brief description
For improvement within a population.
Selection based on the phenotype of
individual plants ; open-pollination.
All plants in the population allowed to
produce polien ; open-pollination.
Inferior plants detasselled ; open-polli-
nation among the remaining plants
Field divided into small plots of about
40 plants each ; selection within small
plots ; open-pollination ; selection
usually for one sex
Plants of a constant genotype (single
cross, inbred) used as check and planted
after every 2-4 hills for comparison ;
check plants detasselled ; others open-
pollinate.
Selection based on means of individual
plant progenies or families.
Plants within each family (individual
plant progeny ) are half-sibs, /.«?., have
one parent (usually the female) in com-
mon.
Families produced by open-pollination ;
selection within superior families ; no
replicated trial ; unrestricted open-polli-
nation among all the families.
As in ear-to-row ; superior progenies
identified by replicated yield trial j pol-
len source— a random bulk of all the
Tamil »Vc
:
i
304
c. Half-sib selection
d. Modified balf*sib
. e. Broad base testcross
f m Marrow base testcross
2. Full-sib
a. Fuii-sibs intermated
b. Si progenies intermated
3. Inbred or Seifed
E» Si
b. S 2
INTERFOP ULATION
IMPROVEMENT
A. Half* sib reciprocal
recurrent selection
(RS-RRS)
B. Full-sib reciprocal
recurrent selection
(FS-RRS)
Plant Breeding : Principles and Methods
As in the modified ear-to-roWi but only
suDerior progenies planted to the cross-
ing block allowed and to open-pollmate.
Half-sibs used for yield trial : S, families
from plants producing superior half-sibs,
intermated through open-pollination.
Half-sib families produced by crbssjng,;
the selected plants to a tester a
broad genetic base (parental or unrelat-
ed) usefi for yield trial ; Si progenies
from olants producing superior half-sib
fSSies intermated (Syn., recurrent,
selection for GCA).
As in the broad base testcross, but the
tester has a narrow genetic base (Syn.^
recurrent selection lor jcaj.
wonts within each family are lujl-sibs £
produced by mating the selected plant*
in pairs.
Full-sibs used for yield trial l ■ superior
full- si bs intermated.
Full sibs used for yield trial : Si pro-
genies from the plants producing supe-
rior fuii-sibs intermated.. ^
Families produced by selling.
Families produced by oae generation of
selling used for evaluation 'superior
families intermated (Syn„ simple recur-
rent selection)
Families obtained by two generations of
selling used for 'evaluation ; superior
families intermated.
Two ' populations improved simultane-
ously for combining ability wits e&ca-
other. , ,
See the description for reciprocal recur—
rent selection given in the text, J t wo
modifications suggested by patevmam
(the' second modification requires at
least- two ears/ plant)
Each selected plant in the populations h.
and B is seifed. Each selected piant
from A is- test crossed with one selected- ■
plant from B» The testcrossed progenies-
evaluated in field trial . Si families of
plants from A producing the superior
testcross progenies intermated ; the same
fs done ' for those from B. Require at
least two ears/plant in one oi the two
populations.
Modifications of Mass Selection , , . . ,
The twa defects of mass selection, viz., (0 lack of contr -
the pollen source and (2) the confusing effect of environm
phenotypes of individual plants, may be corrected by suitable mom
fications of the selection scheme ; these modifications are
described below.
Population improvement
1. Inferior plants in the field are detasselled, and the remaining
plants are allowed to open-pollinate. This modification exercises
some control on the pollen source, but the identification of inferior
plants, of necessity, is based on only those characters which are
expressed before flowering.
2. Pollen from all the selected plants is collected and bulked ; this
pollen is used to pollinate the selected plants. This scheme
ensures full control on the pollen source, but it can be applied
to only those characters which can be selected' for before pollen
shedding.
3 . Stratified Mass Selection. This modification, suggested by
Gardner in 1961, is also known as the grid method of mass selection.
The field from which selection is to be done, is divided into several
small plots, e.g., having 40-50 plants each. Equal number of
superior, plants are selected front each of the plots, Le., selection is
done within the plots and not among the plots. The seed from, all
the selected plants is composited to raise the next generation. The
basis for this modification is the consideration that variation due
to environment, including heterogeneity in soil fertility, will be much
smaller within the small plots than that in the whole field. , Thus
selection within the plots is expected to be more effective than that
without any stratification. Stratified mass selection has been able
to increase "the yielding ability of an open-pollinated variety of
maize, Hays Golden, by about 3% per cycle (or generation) for 15
generations.
4. Plants of a constant genotype, e.g., a single cross hybrid, are
planted as checks after every one, two or four plants of the variety
under selection. The yields of the plants under selection are
expressed as per cent of the yield of the nearest check plant. This
scheme is designed to minimise the environmental influence on the
yields of the plants being selected. It employs the principle
of contiguous control discussed earlier in some detail (Chapter 12).
Effectiveness of Mass Selection
The results from selection studies using mass selection have
been summarised by Hallauer and Miranda (1981). Mass selection
has effectively improved characters with high heritability in maize,
e.g , ear height, lodging resistance, ear type, adaptiveness, oil and
protein contents, resistance to Helminthosporium. leaf blight, days to
flowering (tasselling as well as silking), prolificacy and stalk volume,
and in other crop species. Mass selection, particularly with stratifi-
cation, has also been effective in the selection for increased as well
as decreased yields in maize. The results from 26 studies on mass
selection for higher yields are summarised in Table 18.2. The yiefd
increased at an average rate of 3.5% per cycle of selection,; the
increases ranged from — 1.0 to 19.1 per cent/cycle, the most common
estimates being 1.1 to 3.4% per cycle. The gains under selection
306
Plant Breeding : Principles and Methods
for other quantitative characters, e.g., prolificacy and ,®* r
height, appear to be relatively more than those for yield possibly
dueto a lower heritability for the latter. Selection fw prolificacy,
i.e., increased number of ears per plant, as well as that for single-
eared plants is highly effective in changing the yieldt of the selected
populations as well. It is not surprising since polificacy has a high
positive correlation with grain yield in maize.
Tab!® 18.2. Effectiveness of mass selection in changing yield and yield traits
* in maize.
Criteria of Number of
selection reports
Selection
cycles
Change In the character under selee*
tioni cycle (%)
Mem
Mange
Most common
estimates
Increased
DeeMita
Increased
Beestawd
Increased
Decreased
Increased
Decreased
Increased
Decreased
Yield
26
2-15
3.5 -1.0-19.1
LI-3.4
2
1
8.2 0.7 & 15.7
Prolificacy
1!
Ml
4.7 2.0-11.4
2.0-4.4
2
1 & 11
4.5 1.5 & 7.5
Ear Height
2
5 & 6
5.2 cas 2.5 & 7.9 cm
2.0-3.2 cm
6
4-12
3.5 cm 2.0-7.9 cm
Ear Length
.1
10 '
1.6
1
10 3.2
100-Seed Weighs
1
9
2.0
1
9
2.0
SELECTION WITH PROGENY TEST
Progeny Selection
The simplest form of progeny selection is the ear-to-row
method winch has been extensively used in maize. This method was
developed by Hopkins in 1908. In Its simplest form, the ear-to-row '
method of selection’is as follows (Fig. 16.2, Scheme I).
L A number (50-100) of plants are selected on the basis of their
superior phenotype. They are allowed to be open-pollinate, end
the seeds from individual plants are harvested separately
2. A single row of 10-50 plmts, Le., a progeny row, is grown from
each selected plant The progeny rows are evaluated for desirable
characters and superior progenies are identified.
Population Improvement
SCHEME?
o o o o oo
first o o o o o o
YEAR O O O O CO
o o o o o o
ORIGINAL
population ■ (i) Small progeny rows grown from
1 the selected plants
(ii) Superior progenies identified
(iii) Phenotypicalfy superior plants
, selected from the superior «ro-
mcouo I I | I J | I | I r genies Pr °
1 (iv) Plants allowed to opea-pollinafe;
seed harvested separately
(0 As in items (i) to (iv) in the } Third selcc-
second year i tion cycle
(II Plants selected on the bails of
phenotype
(ii) Open-pollinated seed harvested J
separately >
I Second
f selection
I cycle
VHiRO
mm
MAY BE REPEATED
ONE OR MORE TIMES
YIELD TRIALS
(1) Same as in items (i) and (ii) of
scheme I
o o o o o o
O O O O O O
tS ooooo'o
ooooo o
(I) and (ii) As in scheme I
(iii) The remaining seed from the
plants producing superior progenies - 1
bulked to raise the next generation u
'SECOND
war
(i) Plants allowed to open»pollinate
(ii) Plants with superior phenotypt
selected and seed harvested sepa*
rately
Second
selection
cycle
third
YEAR
■FOURTH
VEAh
MAY BE REPEATED
ONE OR MORE TIMES
YIELD trials
Fig. 18.2. Ear-to-row method of progeny selection as applied to maize. These
schemes are termed as recurrent selection by many plant bleeders
who apply the term to any selection scheme which has two or more
■ ■selection cycles. '
3Qg plant Breeding : Principles and Methods
item 2, are grown from .no adeoted
his sr ":ss,e, y r r
tke JSm* u of on« f «r o S Howover, i. «£« from ,k,
defect that the P“ 5's Ifwelt This redacts to
riST Son > ««« «■*«■ ,he f ' ,,iow " s
££?£££ SVZ£Z 2 * of phenotype (as
in niHtSraSTSS^ s‘ow“S SteS f above)
^"evaluated- The remaining seed from the selected piants ss kept
separately. Superior progenies are identified.
T tod Year The remaining seed from the plants that produced the
loedor progenies (identified in the second year) is bulked o produce
KertVneration. Plants are allowed to open-pollmate A
lumber ofplants are selected on the basis of phenotype and .he
selection cycle' may be repeated one or more times.
This modification of ear-to-row method was. widely used m
breeding of maize in U.S. A. and was responsib e for the develop-
St of several varieties. In this method, plants from superior
progenies only are allowed to mate among themselves. But for each
selection cycle, two years are required as compared to only one in
the case of ear-to-row. method.
Modifications. Several modifications of ear-to-row method ot
selection are available and many more may oe devised to suit the
specific needs of breeder. Some of the modifications are briefly
described below.
1 Seeds for progeny testing are obtained by selfing the selected
liJts and not from open-pollination. This variation is the basis
fi? the simple recurrent selection, and is also known asS, /awftj-
selection. When the seeds for progeny test are obtained after
two generations of selfing, the scheme is known as- S 2 family
selection. . . ,
2 The seeds for progeny testing are obtained by crossing the
selected plants to a common tester. The common tester may be an
open-pollinated variety, a hybrid or an mbred. This variation nas
been refined as recurrent selection for general and specific combining
abilities, included in the half sib family selection, and reciprocal
recurrent selection. P
3 The progeny test consists of a replicated yield trial m place of
a single row In this method environmental, effects can be
separated and the actual value of each progeny can be more
accurately estimated. This modification was proposed by
Tonnauist in 1964, and is by far the most successful method of
reputation Improvement 30!*
progeny selection. In this case, progenies from the selected
• plants are planted in a replicated yield trial as well as in a
crossing block (recombination or seed production plot). The
progenies in the ^ crossing block are detasselled ; they .are
pollinated by the pollen from the rows of a random bulk of all
the selected progenies planted after every 2-3 progeny rows, Superior
progenies are . identified on the basis of the yield trial. Best
plants from the superior progenies in the crossing block are selected*
and their seeds are harvested separately. Progenies from the selected
plants are handled in the same way as outlined above. In. this
•scheme (1) the evaluation of progenies is based on a replicated
trial, (2) the source of pollen is controlled, and (3) each selection
cycle is completed in one year. * This scheme is commonly known .as
modified ear-fo~row method .
4. Seeds for progeny test are obtained by mating the selected, plants
in pairs so that the plants within a progeny are full-sibs, Le., have
both the parents in common. This is commonly known as full-sib
family selection .
Merits of Progeny Selection. The progeny selection schemes, except
those for recurrent selection, have the following advantages.
1. In progeny selection, the selection is based on progeny test
and not on the phenotypes of individual plants. The progeny, test
is a far more accurate reflection of the genotype than the phenotype.
Thus, progeny selection is far more efficient than mass selection in
the identification of superior genotypbs. This is a powerful tool. , for
increasing the yielding ability of open-pollinated varieties of maize ;
the yielding ability increased at the rate of 3-8 per cent per selection
cycle in the different experiments.
2. Inbreeding may be avoided if care is taken to select .a sufficiently
large number of plant progenies and if the selected progenies are
not' closely related.
3. The selection scheme is still relatively simple and easy ; but some
of the modifications are more complicated and tedius.
Demerits of Progeny .Selection. The progeny selection schemes other
than recurrent selection suffer from the following defects.
1. In most progeny selection schemes, there is no control on polli-
nation and plants are allowed to open-pollinate. Thus the selection
is based on the’ maternal- parent only. This reduces the efficiency of
selection.
2. ’ Many of the progeny selection schemes are complicated and
involve considerable work.
3. The selection cycle is usually of two years, Le. the complete
selection ■ process takes two years. Thus the time requirement
for, selection is twice as much as that in the case of mass
selection.' .
Applications of Mass and Progeny Selections. Mass selection ' has
been extensively used for the improvement of cross-pollinated
riant Breeding : Principles and Methods
crops. Eves m the case of maize, where it was almost completely
replaced by heterosis breeding, there is a renewed interest in popula-
tion improvement. Heterosis breedings as would be seen later, is
.expensive, time and labour consuming, complicated and requires
strict pollination control. As a result, in a vast majority of cross-
pollinated crops heterosis breeding is not likely to become economi-
cally feasible. In such cases, mass selection or one of the other
population improvement schemes, will remain the common breeding
method. Mass selection has been used to maintain purity 1 of the
varieties of cross-pollinated crops ; in some crops, progeny selection
h used for the same purpose. The population improvement schemes
have been and are being used for the development of new improved
open-pollinated varieties of many cross-pollinated crops.
Achievements with Mass and Progeny Selections. Mass and progeny
selections have been extensively used for the improvement of cross-
pollinated crops. The early varieties of feajra were developed through
mass selection. -Some of the 'examples are- Babapiiri, Jamnagar
Giant, AF 3, S 35.0 and Pusa’Moti; all these varieties were isolated
from African introductions. Pusa Moti (developed at IARI, New
Delhi) has a wide adaptability and was ■ a popularly grown variety.
Mass selection Improved the yielding ability of toria (B« campestris
var. toria) by 30 per cent and oil content by 56 per cent. ; a further
Increase of 16 per cent in yield was obtained by using mass.- pedigree
method. Many early, erect to'semfefcect types have been developed
Is rai (BJuncea, e.g., Type It, L16), toria {e.g. 9 Abohar), yellow
sarson (B. campestris var. yellow sarson. e,g.„ T 42, T 16) ; and
brown sarson (£. campestris var. brown sarson, e.g. 9 17 dwarf, 17
medium, BS 1 and DS 2). Selections DS 1 and B8 2 gave 2 51 and
286 per cent more yield than the local variety, - were earlier by 5-10
days and had larger seeds. They can be planted as late as the first
week of December.
Varieties have been developed through mass selection in maize
. («•*. T.41, T 1% Imnpmi etc.), cotton [e.g. t C 402, C 520, € 128!
md K 12 from desi cotton (G. arboreum), and 1 CO F, 216 F and A 19
from American cotton (G. hirsutum)], castor (R, communis # e.g S 20,
■ B I and B 4) and in many other crops. A large proportion of the
present area under maize is planted to open-pollinated varieties which
are the products of mass selection. For example, ■ Jaunpuri maize is.
very popular in areas around Jaunpur and Is a successful open-polli-
itated variety. These examples are a small sample of the vast range
of achievements through mass selection.
Becnrrent Selection
The idea of recurrent selection was first suggested by Hayes
and Garber in 1919 and independently by East • and Jones is 1920,,
However, cohesive breeding schemes of recurrent selection were
developed during 1940s, particularly after 1945 when Hull suggested
mat recurrent selection may be useful in improving specific combin*
mg ability. The recurrent selection- schemes were devised in relation
ensure me isolation of
s subjected to recurrent
the production of hybrid
ading inbred line depends
m of superior genotypes
inbreds are isolated, and
the inbreeding process in
s or gene combinations,
at selection is ineffective
en with mild inbreeding*
>t hope to isolate an out-
ny appreciable frequency.
*eeder is to increase the
s using a breeding scheme
5 variations of progeny
of obtaining the progeny
f ie intercrosses among the
■ The recurrent selection
:ed for a specific purpose*
selection, (2) recurrent
RSGCA), (3) recurrent
CA), and (4) reciprocal
rent selection, a number
cted and self-pollinated,
selected plants in the next
in all possible combina-
each cross is composited
etes the original selection
able plants .are selected
. , , „ om the original selection
cycle, they are selected on the basis of phenotype and arc self- /
pollinated. Progeny rows. are grown and all possible, intercrosses are
made by hand. Equal seeds from all the intercrosses are composited
to produce the next generation. This constitutes the first recurrent
selection, cycle.- The population may' be -subjected to one or more
recurrent selection cycles (Fig. 18*3). * ‘ '
In case the character or characters under selection can be easily
and accurately measured on individual plants which are selected and
selfed, the above scheme is followed as such. Some other characters
nd protein contents. In
. plants are evaluated for
rom the plants that are
id fb'f i
IWT
SECOND
YSA «
THIWO
FOURTH
YEAR
OR»G'NAu
POPULATION
INTERCROSS
BLOCK
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COMPOSITE OF
INTERCROSSES
INTERCROSS
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Plans Breeding : Principles and Methods
MAY BE REPEATED
lABm'THE FIRST RECURREN1
SELECTION CYCLE):
(I) Several pbenotypicalfy supe-
rior .plants' selected
(ii) Selected plants self-pollinated
(in) Seed harvested separately
(iv) Seeds' evaluated ; superior
seeds retained
t Original
r Selection '
cycle '
(I) Individual plant progenies
• planted
(ii) All possible intercrosses made
(iii) Equal amounts of seed from,
all intercrosses composited
(i) Composited intercross seed
planted
(ii) As in’ (i) to (iv) in the first
year'
First
j recurrent
l ■ selection
t cycle
^ (i) Individual plant progenies
' planted
(ii) &$ in (Ii) and (If!) in the
second year ■
Elg. 18.3. Simple recurrent selection when the characters under selection can
be easily evaluated on the basis of phenotype of the selected plants
or from the selfed seed obtained from them*
IS very similar to the Scheme I of progeny selection (Fig. 18.2).
However, it differs from the Scheme I in two important ways :
• S o l cte “ ?J. ants are self-pollinated as compared to open-pollina-
tion m Scheme I, and (2) the progenies are intercrossed in all
possible combinations and not open-pbllinated as in Scheme I.
J he S ^ e T e be modified to include progeny test for
pollinated 7)2 f W hen * abli3t /’ yield, A portion of the seif,
pollinated seed from the selected plants is used- to plant progeny
Population Improvement
4
k
rows, preferaWy replicated, which are evaluted for the character
under selection and superior progenies are identified. Next yedr ' ft*
remaining selfed seed from the plants producing superior progenies'
is planted in progeny rows for intercrossing. All possible intercrosses
are made by hand. Equal seed from each intercross' is mixed together
to raise the next generation. This modification is verv similar to the
Scheme II of progeny selection (Fig. 18.2) ; the differences between
the two are the same as those noted above.
Recurrent selection is effective in increasing the frequency of
desirable genes in the population, it is the most suited for characters
with high heritability. The mean of the selected population shifts ii>
the direction of selection. Generally, there is no appreciable reduc-
tion in variability, and in some cases selected populations may show
a relatively larger variation than the original population. Simple
recurrent selection is considerably more efficient than selection with
self-pollination. • There is a relatively low inbreeding, and if care is
taken it can be kept to a minimum. Inbreeding can be avoided in
one of the two ways : (1) the population derived from the mixture of
intercrosses may be allowed to mate at random for one generatibn.
and this open-pollinated seed should be used to establish the popu-
lation for'reselection. Alternatively, (2) each intercross may be grown
separately and it should be ensured that the selected plants are nbt
related by parentage or by des'cerit, /.<?., the plants are not selected
from a few of the intercross only.
Recurrent Selection for General Combining Ability (RSGCA). In
cash of recurrent selection for general combining ability, the progeny
for progeny testing are obtained by crossing the selected plants to a
tester strain with a broad genetic base. A tester strain is the
common parent mated to a number of Maes, strains or plants ; such
a set of crosses is used for the estimation of combining ability of the
lines or plants. A tester with a broad genetic base implies a popula-
tion that has a large genetic variation, 'e.g., ah open-pollinated
variety,' a synthetic variety or the segregating generations of a double
or a multiple cross. Since the gametes from such testers would Be
Variable, the differences between plant X tester progenies would be
primarily due to the genera! combining ability (GCA) of the plants
(the tester being common to all such progenies). It is, therefore,
assumed that the plants selected on the basis of the superior perfor-
mance of their plant x tester progenies would have superior GCA.
RSGCA is a direct outgrowth of the early testing suggested by
Jenkins in 1935. Early testing is the testing of inbreds for combining
ability iti the early stages of inbreeding, e.g., in the first or second,
selfed generation. In 1940 Jenkins proposed a scheme for developing
synthetic varieties from short-term inbreds ; this scheme is essentially
RSGCA. The various • steps involved in RSGCA are schematically
represented in Fig. 18.4, and are briefly outlined below.
First Year. A number of phenotypicaliy outstanding plants are
selected from the source population. The source population may be
an open-pollinated variety, a synthetic or an advanced generation of
m m
314
Plant Breeding : Principles md Methods
a hybrid. Each selected plant is selfed as well. as crossed (as male)
to a number of randomly selected plants from a tester with broad
genetic base. The selfed seeds are harvested separately and saved for
planting in the third year. The test-cross progeny fplaat X tester
progeny) from each selected plant is harvested separately and used
for a replicated yield trial in the second year.
Second Tear . A replicated - yield ' trial is conducted using the
plant x tester progenies. The superior: progenies are identified.
Third Year . Selfed seed (from the first year) from these plants that
_ produced superior test-cross progenies (os’ the basis of the yield
"trial of -the second year) is planted in separate progeny rows in - a
crossing block. These progenies are intercrossed in ail possible
combinations. Equal amounts of seed ■ from each intercross is
composited to raise the next generation. This completes the original
cycle of selection.
Fourth Year . The seed obtained from bulking of ail the intercrosses h
planted as the source population for the first cycle of recurrent
selection. Several plants are selected-on the basis of their phenotype ;
they are selfed as well as crossed (as males) to a number of random
plants from a tester with a wide genetic base.
Fifth Year . Operations of the second year are repeated.
Sixth Year . Operations of the third year are repeated* This completes
the first recurrent selection cycle.
Seventh Year . The second recurrent selection cycle may be initiated
as in the case of the first recurrent selection cycle in the fourth year.
In this manner several cycles of recurrent selection may be done.
Experimental, evidence shows that RSGCA is effective in
changing the GCA in the direction of selection. In addition, it is
also effective in -increasing the yielding ability of the population
obtained at the end of ihg selection cycle. A.t the end of each recur-
rent selection cycle, the population is made up of equal 'amounts -of
seed front all possible intercrosses among a number ‘of progenies
selected on the basis of their general combining ability (Fig, 18.4).
Thus such a population is identical -with ' a synthetic variety and Is
often referred to as such. The yielding ability of a synthetic variety
developed through recurrent selection may be further improved
through additional cycles of selection. r Generally, there is some dec-
reasein # the variability of the selected populations,, most likely due to
inbreeding. This loss in variability could, be prevented if 'care is taken
to avoid excessive inbreeding (see simple recurrent selection), . How-
ever, in the case of characters governed by a few major genes, it may
be expected that the genes would be close to fixation . after, a few
cycles, of selection leading to a drastic decrease in the variability for
such characters.
RSGCA can be used for two basically different purposes*
(1) It may be used to Improve the yielding ability and the agronomic
population Improvement
315
«fl$r
YEAR
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ORIGINAL
POPULATION
SECOND
YEAR
TESTER
<CP£N-PGLUNA TED
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SELF— '
POLLINATED
SEED
FOURTH
YEAR
INTERCROSS
SLOCK
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riFTH SELF-
YEAR
! OOOOOOOOj
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TESTER
TEST 1 CROSS
POLLINATION
INTERCROSS
SLOCK
(i) Several plants selected'
on the basis of phenotype
00 Selected plants self-
pollinated, and
(iiO Test-crossed m males
to a number of randomly
selected plants from a
tester -with a broad genetic
base
(0 Test-cross progeny
from each selected plant
harvested 'separately '
(ii) Replicated yield trial
is conducted ■
(in) Superior, progenies
Identified
(!) Selfed seed from plants :
producing superior- test- |
cross progenies' planted in
& crossing block ?
(ii) All possible later- j
crosses made, and f
(III) Equal seed. from all J
the intercrosses coeapo- (
sited j
(i) Composited seed from i
the Intercrosses planted j
(si) As in (I) to (ill) in tbs i
first year
t
Cl) As in the case of
second year stems (i) ' to
(Hi)
(i) As in the case of
third year Items © to (ill)
I 3*
'ii
! Is
ft g
/!■ ' ':■■■
- MAY BE REPEATED
&&S IN .THE FIRST RECURRENT
SELECTION C YCLE)
j 8,4. Recurrent selection for general combining ability. In the case of
rncmr cut. selection for specific combining ability, an . inbred is used
:&s a tester In. place of m ope&*pollinated variety ; the .rest of the
scheme remains the tame.
Plant Breeding : Principles and Methods
characteristics of a population. In this case, the end product of
selection is used as a synthetic variety. (2) Alternatively it may be
used to concentrate genes for superior GCA. The end product of the
selection is then used for the isolation of outstanding inbreds with
superior GCA. It is expected that the frequency of such inbreds
would increase after a few cycles of RSGCA.
Recurrent Selection for Specific Combining Ability (RSSCA). Recur-
rent selection for specific combining ability was first proposed by
Hull in 1945. The objective of RSSCA is to isolate from a popu-
lation such lines that will combine well with a given inbred. It is
assumed that a large part of heterosis is the result of nonadditive
gene action, i.e. 9 dominance and epistasis. This part of heterosis
will, therefore, depend on specific gene combinations and is desig-
nated as specific combining ability (SCA). If plants are selected on
the basis of the performance of their progeny derived from The test-
cross with an inbred, they would be selected for their combining
ability with, the inbred used as tester. It may be expected that these
plants wouM have genes or gene combinations that specifically
combine well with the genes present in the tester inbred. The
procedure for RSSCA is identical with that for GCA, except that
in this case an inbred is used as a tester In the place of an open-
pollinated variety. The tester must be an outstanding inbred
because it would be one of the parents of the hybrid that would be
produced using the inbred lines isolated from the improved
population. Therefore, great care must be exercised in the selection
of the inbred to be employed at the tester in a- RSSCA programme.
The selection scheme is briefly outlined below (Fig. 18.4).
While referring to the Fig. 18.4, the tester should be read as an
outstanding inbred.
First Tear. Several plants are selected from the population and
self-pollinated. The selected plants (used as males) are also crossed
to an outstanding inbred used as tester (used as female).
Second Year . A replicated yield trial is planted using the testcross
progeny. Outstanding progenies are identified.
Third Year. Sejfed seeds from the plants that produced the out-
Population Improvement 317
objective of RRS is to improve two different populations in their
ability to combine well with each other. In this scheme, each of
the two populations A and B serve a$ testers for the plants selected
from the other population. For example, a random sample of plants
from population A serves as the tester for the plants SQkcitd from
population B ; similarly, a random sample of plants from population
B serves as the tester for those selected from population A. It may
be seen that this selection method allows for the selection for both
GCA and SCA. _ It selects for GCA because the two testers (popu-
lations A and B) have brpad genetic base since they are genetically
heterogenous. Selection fo'f..SCA is accomplished because ' the two
populations (or inbreds derived from them) would be crossed with
each other to produce the commercial variety, and the plants in each
jbf the two source populations are selected for their abilities to com-
bine well with the gene combinations' present in the other popula-
tion, that is, for SCA with each other. A generalised scheme for
RRS Is outlined below (Fig. 18.5). ..
First Year. Several plants are selected from the populations A and B
on the basis of their desirable phenotype. Each of the selected plants
from population A is crossed as male with several randomly selected
plants from the population B used as female. Similarly, each of the
plants selected from the population B is crossed as male with a
random sample' of plants from the population A 'used as femaje. All
the selected plants are selfed ; the seifed seed is harvested separately
and is stored for use in the third year. All the testcross seed from
an individual plant is composited and is used for progeny test in the
second year (Fig. 18.5).
Second Year . Two replicated yield trials are conducted : one trial
Is for the testcross progenies of plants selected from the population
A, and the other one is for those selected from the population B. On
the basis of the progeny tests, plants producing superior progenies
are identified.
Third Year . Selfed seed from the plants selected on the basis of
the 'progeny tests (in the second year) is planted in two separate
crossing blocks in individual plant-progeny rows. In one crossing
block, seeds from the plants selected from the population A arc
planted, while, in the other crossing block seeds from the plants
selected from the population B are sown. Within, each crossing
block, all possible intercrosses among the plant* progenies are made.
Equal amounts of .seed from all intercrosses from the crossing block
A ‘are mixed to raise the next generation of the population A.
Similarly, the next generation of population B is raised from the
seed obtained by mixing equal amounts of seed from all possible
Intercrosses from the crossing block B. This completes the original
selection cycle .
Fourth Year . Populations A and B are planted from the composited
seed from all the intercrosses in block A and B, respectively.
Operations of the first year are repeated.
Fifth Year . Operations, of the second year are repeated.
318
Plant Breeding :
FIRST
YEAR
ORIGINAL
PORULATIONCA)
. ommnM,
POPULATION l&\
Principles . Methods
<i) Several plants!
selected in the popu- i
Sations A and B !
(ID Selected plants: j
self-pollinated
(ill) Bach selected
plant from A is test- ,
crossed with several
random plants from
B, and vice-versa.
I
<i) 'Test-cross . pro-
geny harvested sepa-
rately
• (ii) Separate yield'
trials conducted for
test-cross progenies
from .populations A
and.B
(HO Superior pro-
genies identified
(I) Selfed seed from
plants ■ producing
superior test-cross
progenies planted
(ii) All possible inter-
crosses are made
(Hi) Seed from aSi
intercrosses of a’
population mixed
(i) Composite seedl
from, intercrosses-
planted separately
(ii) As in the first,
year items (I) to (II)
(I) As in the second
year items 0 ) to '(III) I
(0 As in the third
year items (!) to (in)
©
I
J
SEPARATE INTERCROSS BLOCKS
t »
may BE REPEATED {AS IN THE
FIRST RECURRENT SELECTION CYCi E)
Fig* 18.5. Reciprocal recurrent selection. Two populations with broad genetic
base serve as testers for each other.
First recurrent selection' cycle.
Population Improvement
3 19
Sixth Year. Operations of the third wa ,
tes the first recurrent selection cycle 'The This comple-
jected to further selection cycles may fae sub ~
operations of the first recurrent selection cyde. by repeatin S
the fo!SwS° “t“ aS deVd0pcd by RRS be used in one of
’• P?P "“ M wi,h
in this case the populations have been Ik- V . ar f taI cross > but
for combining ability (both GC 4 and SPA V** Ct * t t0 se3ecti °B
In maize (Z. mays), crosses Swm ™ ^ . Wlth each other,
reciprocal recurrent selection* shoV P h£hll?° D ?w 3m P roved ^
between the original populations (set latff) ^ ^ **“ thbSe
2 -
single cross. In this case *he inh rid® d £ Ce m d ° u b 3e cross or a
following order? ’ Iabrcds shcmld be crossed in the
Single cross .-(AiXBi)
Double cross : (AiX AsJxfBjXBg)
X Md b‘x S b! S KKiS 1 ?”" i te p***"
B - T j» s would permit the maximum expreSSJ ^*i 1 popul ? ti< ? n
the double cross. There is considerab^??^ L beter ? s,s
recurrent selection increases the yielding ability of th^Kidi
P S te ? ib,<*« B ,d 1 . hkrai , tionfJt* £ S22S
Plant Breeding : Principles ana Methods
Effectlfesess of Progeny Selection Schemes
There is considerable literature on the effects of. ^ Yarious
orogeny selection schemes on yield and yield traits in maize and
other crop species. Much of the information concerning maize has
Sen reviewed by Hailauer and Miranda (1981), and is summarised
In T-ibl° 18 3 Ear-to-row is the earliest progeny selection scheme ,
U effectively “improved highly heritable traits, e g., oil and protein
contents but failed to improve yielding ability, ibis failure was not
so much due to the deficiency of the selection scheme as due to
ft) ooor field plot technique during selection and evaluation, (2)
contamination from foreign pollen, -and (3) inbreeding depression
due to small experimental populations- A welcome outcome of
this failure was the developement ot the hybrid varieties. The modified
ear-to-row scheme proposed by Lonnquist in i9o4 is highly effective
ia improving the yield of maize (average gam 4.8 % per cyme).
Full- sib-family selection has been evaluated rather extensively,
and is extremely effective in improving maize yields (average gain
5 3°/ per cycle). Inbred (S, and 3$) family selections are also effective
in improving maize yields, but the rate of change per cycle appears
to be somewhat less than that for modified ear-to-row and full-sib
family selections (average gain 4.6 and 2.0 per cent/cycle for Sj and
g„ respectively). Recurrent Selections (RS)for GCA and SC A effec-
tivelv improved the yields of the poulations themselves (average
gain 30.2 and 5.2% per cycle, respectively). These schemes also
improved the GCA (measured as the yields of testcrosses) of the
populations, the rate of change being slightly higher for RSGCA
than that for RSSCA. Reciprocal recurrent selection ( RRS ) improved
the vieldin® ability of the populations themselves as well as that of
the population crosses. Some investigators, however, have reported
mprovement in the yields of the population crosses only, while those
3 f the populations per se were not affected. Use of full-sibsjn
RRS appears to be relatively more effective than the use of half-sibs
in* improving the yielding ability of the two populations, but in
improving the yields of the population crosses the latter appears to
be more effective.
The results from different selection schemes (Table 18.3) are
not strictly comparable with each other for the following reasons.
Firstly , the different schemes were applied to different populations,
and the response to selection may vary greatly from one population
to another. Secondly, the selections and evaluations were made
during different years and at different locations. Thus the E and
GxE components of variation will be different for the different
studies. Lastly, the intensities of selection were different in many of
these studies, and the effectiveness of selection is greatly affected by
the selection intensity. For these reasons, the results from different
selection studies should be compared with considerable caution.
Comparative Effectiveness of Different Selection Schemes
On the basis of th*nr»»tWl Avneetations. the following eeneraii-
Population Improvement
321
sations can be made. (1) The gain under selection would be propor-
tionate to the magnitude of <r*A in the population suWe&eT to
se ection, i.r„ the higher the c*A, the greate? the gain unSeSion
(2) As a consequence, the gams from inbred family selections w2i
SiTn/eV t, 0S ,f f f° m ful! ; sib selection, which would be gTeaS
than those from half-sib selection schemes. ® cr
Meaningful comparisons among some selection schemes are
available from 13 studies reviewed by Haliauer and Miranda n 08 n
.ndare yarned i„ Table 18.4. In .base “ Ss J mlli
selection schemes were applied to the same population under £>m
parable experimental and environmental conditions. Recurrent sec-
tion for GCA may be based on an unrelated tester or the parental
population itself may serve as the tester. The use of the parental
population as tester seems to be superior to the unrelated teste ? In
improving the performance of the population perse, but it appears
to be inferior to the latter in increasing the yielding ability of the
A comparison is available between RSGCA and
RSSCA » tie results from the four studies are contradictofy. In two
studies, RSSCA was superior to RSGCA in improving the yields of
the populations themselves as well as that of the testcrosses. But in
tne remaining two studies, RSSCA was superior to RSGCA in
improving the yields of the testcrosses, but it was inferior to the fetter
sn increasing the yields of the populations perse. Thus RSSCA
appears to be superior to RSGCA for improving the yields of the
testcrosses (with several testers) of the selected populations, ie In
improving the GCA of the populations. This seems a little surprising
because a narrow base etester is expected to select primarily for
In tw'o studies, S 2 family selection was compared with RSGCA
and RSSCA ; it was inferior to RSGCA in improving the yields of
the populations per se, and to both RSGCA and RSSCA in increas-
ing the yields of the testcrosses of the selected populations This is
surprising since theoretically S 2 family selection is expected to be
superior to the other family selection schemes. Considerable
information is available on the comparison between RSGCA and S
family selection ; RSGCA w r as marginally inferior to the latter
in increasing the yields of the populations per se, but it was some-
what superior in improving the yields of the testcrosses of the
improved populations. One comparison is available between full-sib
and S t family selections ; full-sib selection was markedly superior to
the latter in increasing the yields of the population per se,
• The effectiveness of full-sib (FS) and reciprocal recurrent selec-
tions^ ( RRS ) was compared by Moll and coworkers in the populations
Jarvis an Indian Chief. Four independent comparisons were made
after 3, 3, 6 and 8 cycles of selection. In general, FS was superior
to RRS in improving the yields of the two populations themselves,
while RRS was superior to FS in increasing the yields of the popula-
lion crosses. After 8 cycles of selection, the average gains per
Plant Breeding : Principles and Methods
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Population Improvement
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Plant Breeding : Principles and . Methods'
election cycle (expressed as per cent of the mean of the original
uiselected populations and their cross) were : Jarvis 3.6 and 2,5,
Indian Chief 3.2 and 1.4 and Jarvis X Indian Chief 2.2 and 3.2 for
T n FS and RRS schemes, respectively.
In Tables 18.3 and 18.4, the gains under selection have been
Impressed on the per cycle basis. However, a more meaningful esti-
mate of the gains would be on the per year basis for obvious
reasons. This is particularly important when comparisons are being
mide among selection schemes that require different number of years
peg selection cycle. For example, modified ear-to-row scheme re-
quires only one year per selection cycle, while most progeny selection
schemes require 3 growing seasons/years per cycle. Therefore, the gains
pet year for the former shall remain unchanged, while that for most
other progeny selection schemes will be only one-third of those listed
in Table 17,3. Clearly the modified ear-to-row method appears to-
be the most 4 efficient selectibn scheme for improving the yielding
ability of maize populations ; the same conclusion is likely to
be valid for other cross-pollinated crops as well.
General Conclusions from Selection Experiments in Maize
'Selection' experiments in maize have generated valuable
information on the effects of selection as well as on the genetic
architecture of maize populations. Some generalised conclusions
bmli on these experiments are summarised below.
1. In general, response to selection for yield varies from one
population to another, and from one selection cycle to the
other. In many cases, the first or the first few selection cycles
are quite effective, while the gains through subsequent
selection cycles are relatively smaller. However, in some cases*
the reverse may be the case.
2^ All the selection schemes cause a marked reduction in the
total genetic variance. This reduction generally occurs during
the first few selection cycles, after which there is little change.
This may explain the decreased effectiveness of the later selec-
tion cycles.
3. The major portion of genetic variance is additive in nature*
and it readily responds to selection. The additive genetic
variance generally shows some decline under selection, but in
some cases it may show an increase.
4. A considerable additive genetic variance remains even after
several cycles of selection so that : (I) there is continued
response to selection, and (2) reverse selection is also effective.
This is most clearly demonstrated by the studies on selection
for high and low protein as well as oil contents in maize.
5. All the selection schemes mainly select for GCA, i.e. 9 additive
gene effects. £ven RSSCA and RRS select primarily for GCA ; .
there is tittle evidence that either of these schemes select for
Population Improvement 325
overdominance gene effects. This conclusion is contrary to the
theoretical expectations as discussed earlier.
6. RSGCA, RSSCA and RRS increase the test-cross performance
of the selected 'populations as compared to those* of the un-
selected controls. However, the increase in heterosis (testcross
performance) is mainly due to the increased yielding abilities of
the populations per se, Thus the change in heterosis of the
selected populations is primarily due to the selection for addi-
tive gene effects.
7. The dominance relationship in the maize populations seems to
be partial to complete dominance. There is little experimental
evidence to support the existence of overdo niinance.
-8- There is some evidence that yield may reach a plateau under
selection. Mass selection (stratified) in unirradiated as well as
irradiated Hayes Golden open-pollinated variety of maize, and
modified ear-to-row selection in unirradiated Hayes Golden
improved the yield by about 40% over the unselected control
after 12 and 6 cycle of selection, respectively. Four additional
cycles of mass and modified ear-to-row selections failed to
improve the yield any further, and surprisingly there was a
-marked decline during the next two selection cycles. The
decrease in the yield under selection is difficult to explain, but
the plateau is reminscent of that for chaetae number in
Drosophila (Chapter 9). It would be interesting to see if
reselection after a period of suspension of selection would be
able to break this plateau : if this fails, means will have to be
devised to achieve this goal for a continued response to
selection.
9. In general, selection for one character, eg., yield leads to
changes in several other characters for which no selection is
done. Such a correlated response to selection is a common
feature of the selection experiments, and k most likely due to
linkage among the genes governing these traits.
10. Some selection schemes, viz., full sib (seifs used for intermating)
and full-sib reciprocal recurrent selection, inevitably select for
prolificacy. Consequently, these schemes would select for yield
, both directly as well indirectly. The indirect selection for yield
is due to the selection for prolificacy which shows a high posi-
tive correlation with yield. Therefore, while comparing ae
effects of these schemes on yield with those of other schemes,
this fact should be .kept in mind.
SUMMARY ..
Population improvement aims at improving the civyr? "ter k tics of a
population through selection. The selection .may he-bused.cn phene type with-
out progeny testing, mass sct?ctk nor on phenotype as well as progeny
testing, e.g., progeny selection and recurrent selection. In nresi seketion,
open-pollinated seed is collected from a number of plants with superior pheno-
type,; seed .from these plants is composited to produce the next, generation* .
additive gene action, (ill) characters governed
characters with high heritability, (v) 'cbarac-
Plant Breeding : Principles and Methods
The selection cycle may be repeated. Mass selection is simple, takes less time
and is highly elective in improving characters with high heritability. Modifica-
tions of mass selection that take into account -the variation due to environment
are effective in improving characters with low heritability as well. However,
it is based on the female parent only.
In progeny selection, plants are selected on the basis of their phenotype
and subjected to progeny testing. The progeny for progeny test may be obtained
by open-pollination, self-pollination, crossing with an open-pollinated variety, a
hybrid or an inbred. Superior progenies are identified ; pheootypicafly superior
plants from these progenies are selected and subjected to. progeny tests. The ■
. *; .-lection cycle may be repeated several times. There are several modified schemes
of progeny selection ; recurrent , selection .schemes are , improvements on these
schemes. Progeny selection is relatively simple, and is based on progeny test,
but there is no control on the pollen parent, and often these schemes take a
longer time than mass selection.
Recurrent selection schemes .case the selection on progeny tests and
exercise rigid control on pollination. The seeds for progeny tests are obtained
by selling (simple recurrent selection), or by crossing to a tester with a broad
genetic base (recurrent selection for GCA) or to an inbred (recurrent selection
for SCA) ; in reciprocal recurrent selection two source populations are used as
testers for each other. The selected plants are selfed as well as crossed to the
appropriate tester. The test cross seed is used for progeny testing. Plants pro-
ducing progenies are identified by progeny tests ; selfed seed from these plants
is planted in a crossing block. All possible intercrosses are made among the
progenies. Equal amount of seed from all the intercrosses is mixed to produce
the population for the first recurrent selection cycle. The first recurrent selec-
tion cycle consists of a repetition of the operations outlined above.
■ Simp. . recurrent selection is effective in. improving characters with high
heritability. Recurrent selection for GCA is highly effective in improving the
GCA as well as the yielding ability of the selected populations, while that for
SCA improves SCA and yielding ability . Reciprocal recurrent selection would
Improve GCA, SCA and yielding ability of the two source population in rela-
tion to each other. Reciprocal recurrent selection is expected to be either
equal or superior to the other recurrent selection schemes. under different genetic
situations, ranging from complete dominance to overdominance. But In most
practical, situations, reciprocal recurrent selection would be superior to recurrent
selections for GCA and SCA.
QUESTIONS
1. Write short notes on the following : (i) mass selection, (ii) progeny test,
Oi*) line breeding, (iv) ear-to-row method of selection, (v) recurrent
selection, (vi) population improvement, and (vii) objectives of selection,
in cross-pollinated crops.
2. Briefly describe the contributions of the following scientists, (i) Hopkins,
(ii) Hayes' and Garber* (iii) East and Jones, (iv) Jenkins, (v) Hull,
and (vl) Robinson, Comstock and Harvey.
3. What is recurrent selection ? List the various types of recurrent selection.
Describe one of the schemes 'for recurrent selection -and compare the
effectiveness of the various schemes,
4. Compare the efficiency of the different schemes 'for population. Improve- -
ment. Which scheme do you prefer and why?
5. In the case of following, what breeding method(s) would be appropriate
and why ? (I) characters governed by dominance -
W characters governed b; ‘
by tphmic interactions, (i
tors with low heritability.
tmm% selection, progeny test/ recurrent selection
mth€A f tester, reciprocal recurrent selection.
Population Improvement
327
% Describe briefly the merits, and demerits, of the following : (i) mass
Selection* (11) progeny selection, (iii) simple recurrent selection, (iv) re-
current selection for SCA, and (v) recurrent selection for GCA,
Suggested Further Reading
Allard, R.W. 'I960, Principles of Plant Breeding. John Wiley and Sons Inc.
New York,
Brown A.H.D, and Allard, R.W, 1971. Effect of reciprocal recurrent selection
for yield on isozyme polymorphism in maize. Crop Sd. 11 : 888-893.
Comstock , R.E. Robinson, H.F. and Harvey, P.H. 1943. A breeds ntr proce-
dure designed to make maximum use of both genera! and speciiic'com Din-
ing ability. Agroo. J. 41 : 360-367.
Center, C.F. 1976, Recurrent selection for yield It* the Fa of a maize single
cross. Crop Sci, 16 : 350-352.
Genter, F. 1976.. Mass selection of a composite of intercrosses Mexican races
of maize. Crop Set, 16 : 556-568.
Hallauer, A.R. 1970. Genetic variability for yield after four cycles of reci-
procal recurrent selection In maize. Crop. Sci, 10 : 482-485.
hallauer, A.R. and Miranda, LB. 198L Quantitative Genetics in Maize
Breeding. Iowa State University Press, Ames, pp, 159-265.
Horner, E.S., Lundy, H.W., Lutrick M.C. and Chapman, W.H. 1973.
Comparison of three methods of recurrent selection in maize. Crop Sci,
1.3 * 485-489,
Penny, L*H. and Eberhart, S.A. 1971. Twenty years of reciprocal recurrent
selection with two synthetic varieties of maize. Crop Sci, 11 : 900-903.
Simmon ds 3 N.W. 1979. Principles of Crop Improvement. Longman, London
and’ New York.
Verhalen, L.M,, Baker , JX. art d McNew, R.W. 1975. Gardner’s grid system
and .plant selection efficiency in cotton. Crop Sci. 15 : 588-591 .
CHAPTER 19
Hybrid and Synthetic Varieties
The 'two important features of cross-pollinated species are
inbreeding depression and heterosis. Population improvement
schemes generally aim at keeping in breeding at a low level to avoid
its ill effects, and an effort to exploit heterosis is rarely made. But
heterosis is the basis for the breeding of hybrid and synthetic varie-
ties. In fact,, hybrid varieties are the best means of utilizing heterosis.
Synthetic varieties utilize only a part of heterosis, but under certain
situations they offer the only practical means for it. Hybrid and
synthetic varieties have’ been highly successful in many cross-polli-
nated species, e.g. 9 maize, jo war and bajra, and in even some self-
pollinated species, e.g., cotton, rice and .tomato. In India, almost
all the recommended varieties of maize today are either hybrid or
composite varieties. As in the other chapters on cross-pollinated cron
species, more is known about hybrid and synthetic varieties in maize
han in ail the other crops combined. Our discussion would
naturally be primarily based on the knowledge gained from maize,
it is equally applicable to other crop species.
DEFINITIONS
h „ iu , a Hy < ridvane ! ks ™ the first generations (Fi) from crosses
between tViO pur elutes, inbreds, open-pollinated varieties, clones or
other populations that are genetically dissimilar. Most- of the commer-
cial hybnd varieties are F x ’s from two or more purelines (tomato,
0r ,abj ' cds / maize > z.mays ; jowar , S. bicolor ; and
ui ■’ ' ■amen.canwn). An inbred is a nearly homozveous line
by Coatinuous Reding of a cross-pollinated spedes vthh
and if maintained by close
and pT* Z’ FT ' r A b f ‘ y by se!f -po i] iaation. When two inbreds, say A
and b, aie crossed to produce the hybrid (A X B), it is known as a
* in sj e cross. Wnen two single crosses, say (A X B) and (CxD) ar*
a i/ e resultl, h? hybrid population, (AxB) x (CxD), is known
douole cross. Thus a double cross involves four inbreds, which
Hybrid and Synthetic Varieties
329
are first mated to produce two single crosses ; the single crosses are
then hybridized to yield the double cross. A threeway cross is a cross
between a single cross (A X B) and an inbred (C) to yield the hybrid
population (AxB)xC. When an inbred is crossed with an open-
pollinated variety, it is known as an inbred-variety cross or a topcross
A topcross, however, also denotes crosses of selected plants, lines or
clones with an open-pollinated variety. The purpose of the top-
cross is to estimate GCA of the plants or lines crossed with the open-
pollinated variety. When the cross is made to assess the combining
ability, it is known as testcross ; a testcross may be made with an
inbred (for SCA), hybrid, synthetic or open-pollinated variety (for
GCA). The common parent used in the test-cross is known as
tester, and the progeny derived from these crosses are known as
testcross progeny. Another term that deserves mention is polycross
Polycross denotes the progeny of a line produced through random
pollination by a number of selected lines. We shall turn back to
polycross in connection with synthetic varieties. When two open-
pollinated varieties are mated, it is known as varietal or population
cross.
HYBRID VARIETIES
History
Hybrid varieties were first commercially exploited in maize.
The use of hybrid varieties was, ironically, not prompted by the
superiority of hybrids, but by the realization that mass and progeny
selections were not able to improve the yielding ability of open-
pollinated varieties to any substantial extent. As early as 1878, Beal
had shown that certain varietal crosses showed substantial heterosis;
he suggested that such varietal hybrids may be used as varieties.
Varietal hybrids were indeed grown commercially on a small scale.
In 1909, Shull suggested that inbreds should be developed from
open-pollinated varieties by continued self-fertilization. The inbreds
that combined to produce superior hybrids should then be crossed
to produce single cross hybrid varieties. Shull’s scheme could not be
exploited commercially because of the following reasons :
1. _ Outstanding inbreds that would produce hybrids with yielding
abilities substantially higher than those of open-pollinated varieties
were ne t available.
2. Since the female parent was an inbred, the amount of hybrid
seed produced per acre was low (30-40 per cent of the open-
pollinated varieties). Consequently, the hybrid seed was expensive.
3. The male parent was also an inbred ; hence pollen production
was poor. Consequently, more area had to be planted under the
male parent. This made the hybrid seed more expensive.
4. The hybrid seed was often poorly developed as it was produced
by an inbred and had a relatively poor germination making a higher
seed rate necessary.
Plant Breeding t Principles and Methods
The last three of the above difficulties were overcome by the
double cross scheme proposed by Jones in 1918. Since hi a double
cross the female as well as the male parents are single crosses, seed
and pollen production is abundant, seed quality and germination are
high, and, as a result, the cost of the hybrid seed is low. The idea of
double cross was adopted soon after it was proposed ; the first
commercial variety was released in 1921. Popularisation of com-
mercial hybrid varieties in U.S.A. was slow, but by 1944 about 80
per cent of the area under maize was under hybrid varieties. It k;
believed that in U.S.A. hybrid varieties represented, on an average
an yield increase of about. 20 per cent over the open-pollinated
varieties. In India, this increase has been estimated to be still higher
vis., 30-50 per cent. ' ’
The utilization of cytoplasmic-genetic male sterility is one of
the significant landmarks in the development of hybrid varieties.
The Texas cytoplasmic male sterility was identified in 1938, and it
was commercially exploited for hybrid seed production during late
1960s. Subsequently, population improvement of the source popula-
tions and the improvement of inbreds markedly increased the yielding
ability of the inbreds. As a result, by 1960 single crosses bad
become widely accepted in U.S.A. at the expense of double crosses.
Today in U.S.A., single cross maize hybrids have virtually replaced
double cress varieties.
In India, intensive research for the development of hybrid
maize began in 1952 when the Coordinated Maize Improvement
Project began in collaboration with the Rockefeller Foundation.
A large number of inbeeds and other germplasm were introduced
rrom U.S.A. ; the inbreds were used for the production of hybrid
maize ( Z . mays). Attempts to isolate superior inbreds from the
indigenous germplasm have not been much successful. Four hybrids
were released in 1961 ; these were Ganga 3, Gasga 101, Raujit and
Deccan. Subsequently, in 1961, work on hybrid varieties in iowar
(6. bicolor j and bajra (P. americanum) was taken up; these* were
***** the cytoplasmic-genetic male sterility cf Combine .Kafir 60
r'crj T’ ft ^ ; res P ec tkely. _ The first hybrid variety of iowar.
CSH 1, was released in 1964, while that of bajra, HB 1 (HB=hybrid
a^ra,/, was released a little later in the same year. However, some
eajra hybrids developed without the use of male sterility were
c? a t r,ie os tl2Sn this l. e t’ hyfcrid Xi and hybrid X a in
, r’i a ! e ’ ^5 per cen ; higher yield over the best varieties),
more'vMd)* 8 ' 15 ' 1 X in 1955 (Maharashtra, 10 percent
Hybrid maize could not become very popular in India for
several reasons. The foremost reasons were the intensive manage-
ment and input requirements, and the need for changing the sfed
every year Subsequently, the development of synthetic ot composite
aneties was undertaken ; in 1967 six composite varieties e g Viiav
* isan ' were Today He emphasis
Hybrid and Synthetic Varieties
31 1
Coordinated Maize Improvement Project is on the development of
composite varieties and on population improvement. But in jowar
# and bajra, hybrid varieties remain the breeding objective.
Operations in the Production of Hybrid Varieties
In the production of commercial hybrid varieties of sexually
propagated species., in bred s are highly desirable in comparison', to
open-pollinated varieties or other populations with a broad genetic
base. This is because (1) inbreds can be maintained indefinitely
without h change in their genotype, while the genetic make-up of
open-pollinated varieties is likely to be modified by the evolutionary
forces" (Chapter 8) ; (2) the hybrids derived from inbreds are
homogeneous, or nearly so, year after year/ while those produced
from open-pollinated, varieties arc likely to be variable, and hence
, their performance cannot be accurately predicted ; (3) the unifor-
mity of the inbred-derived hybrids, is also desirable from the
viewpoint of uniform quality of the produce. For these reasons,
hybrid varieties in case of maize and other crops are produced
almost exclusively from inbred lines. The operations involved in
the production of hybrid varieties in such a case are : (1) develop-
ment of inbred lines, (2) evaluation of inbreds, and (3) production
of hybrid seed (Fig. 19.1),
Development of Inbreds
Inbred lines are developed from a genetically variable popula-
tion by continued inbreeding. The population from which inbreds
are isolated is the source population . The source population is gene-
rally an open-pollinated variety, but it may as well be a synthetic, a
single cross or a double cross. Inbreds isolated from an open-
» pollinated variety, which may or may not have been subjected* To
population improvement, are known as first cycle inbreds . On the
other hand, inbreds isolated from hybrid varieties are termed as .
. second , third or fourth cycle inbreds depending upon the number of
improvement cycles (0, I and 2, respectively) the inbreds making-up
the- hybrid varieties have been subjected to. The inbreds isolated
from synthetic varieties may or may not be second cycle inbreds. If
a synthetic variety is derived from inbreds, the inbreds isolated
from it would-be second or higher cycle inbreds. otherwise not. The
development of second cycle inbreds is considered later under
Improvement of Inbreds’. The production of first cycle inbreds
is briefly outlined here.
Isolation of Inbreds by Inbreeding. Inbreds are developed by a suit-
able system of close inbreeding. But self-pollination is desirable,
r wherever possible, as it. leads to homozygosity very rapidly. The
procedure for the isolation of inbreds through self-pollination is
described below.
First Year . A number of plants with desirable phenotypes are select-
ed from a source population and are self-pollinated. The selected
plants should be vigorous and free from diseases. They may be
selected on the basis of their GCA estimates obtained by testing the-
332
Plant Breeding : Principles and Methods
STEP 1. PRODUCTION
OF INBRED LINES
2c SINGLE-
GROSS TEST
FOR sea
3. HYBRID SEED
PRODUCTION
. SOURCE POPULATION ,
ill*
SELFING OR SOME OTHER FORM
, OF CLOSE INBREEDING
I t f i
infill ilium <
INBRED UNES
I I \
STEP 2 EVALUATION
OF INBRED LINES
2 a, PHENOTYPIC
EVALUATION
2&.TOP -CROSS
TEST FOR gca.
nniHn
iinitm
HI 1 ! 1 1 1 1
iiiiiiiP
llUlllli
UlilM jt
Ifnilill
if 1 i 1 M II
h i inii!
min i tr
1
mini nj
! 1 tl 1 ! 1 1 !
j REPLICATED TRIAL j
I I * ♦
SELECTED INBREDS TOP-CROSSED
WITH A TESTER WITH Wj DE
BASE
REPLICATED TRIAL USING TOP-
CROSS PROGENY
I- I
SELECTED INBREDS FROM EACH
SOURCE POPULATION MATED
SEPARATELY IN ALL POSSIBLE
! I
REPLICATED TRIAL USING SINGLE
, CROSSES
I I I I
OUTSTANDING INBREDS FROM
ALL POPULATIONS ARE MATED
IN ALL POSSIBLE COMBINATIONS
iuuiumiuiiiiimm
ituiuiHimifiititmi
mmmimmmmdf
REPLICATED TRIAL USING SiNGLE
CROSS PROGENY
i
OUTSTANDING INBREDS USED
FOR COMMERCIAL SEED
PRODUCTION
I
SINGLE/ DOUBLE
CROSS
Open-pollinated plants
from diverse source popu-
lations' are subjected to
continuous inbreeding with
selection. After 6-7 genera-
tions of self-pollination,
nearly homozygous inbred
lines are obtained.
Selection among inbreds
based on phenotypic evalu-
ation from a replicated
trial. Very poor inbreds
are discarded.
Inbreds tested for GCA on
the basis of topeross test.
About 50 per cent of in-
breds rejected on the basis
of the performance of top-
cross progeny.
The remaining Inbreds are
subjected to tingle cross
test for SCA. Inbreds from
different source popula-
tions are evaluated sepa-
rately for economy.
Few outstanding inbreds
from each of the popula-
tions are pooled and cross-
ed in all possible combi-
nations. Outstanding single
crosses are used for esti-
mating the performance of
various double crosses.
The final step consists of
• the production of the out-
standing single or double
cross at a commercial
scale.
Fig. 19,1. Steps in the production of hybrid varieties in cross-pollinated crops,
based on the present practice in maize.
Hybrid and Synthetic Varieties
331-
performance of their testcross progeny (the tester should have a
p wide genetic base). Experimental evidence clearly reveals that open-
pollinated (So) plants differ in their GCA and that GCA can be
successfully selected for.
Second Year . About ‘30-40 plants are space planted from the selfed
seed from each of the selected plants. Best plants are selected from
the best progeny 'rows and are self-pollinated.
Third to Sixth Years . The process of the second year is repeated..
But as the number of generations of seif-pollination increases, indi-
vidual plant progenies would become more and more homogeneous.
Consequently, in the later phases of inbreeding selection is primarily
among the progenies rather than within the progenies. •
4 Most of the material would be discarded due to deficiencies,
and weaknesses, but a few outstanding lines would be maintained
These lines would be the inbreds that might be useful in a. hybrid
programme.
Seventh Year. At this stage, individual plant progenies would be
almost homogeneous as they would be expected to be nearly homo-
zygous. Selling may be discontinued and the inbreds are generally
maintained by sib-poiiinatioiL
Development of Inbreds imm Haploid Plants. In maize, haploid
plants occur spontaneously at a low frequency ; the frequency varies-
from 10" 3 in most of the strains to 10~* in certain genotypes. These-
haploids originate due to the parthenogeoetic development of female
gamete. Therefore, the identification of such haploids becomes very
easy if a pollinator carrying a dominant seedling marker is used ;
all the plants in the progeny lacking this marker are likely to' be
haploids. Haploids are sterile, hot about 10 per cent of the haploids
set some seed due to spontaneous chromosome doubling in small-
sectors. The diploid progeny from haploids are homozygous and
serve as instant inbreds. This method presents a short-cut to the
lengthy inbreeding programmes for isolating inbreds. Haploid plants
may also be obtained by parthenogenetic development of the male
gamete, but the frequency of such haploids is very low.
A substantia! number of inbreds developed by the haploid techni-
que has been evaluated for combining ability . They are comparable
to a random sample of the conventional inbreds developed by selfing*
Selection During Inbreeding. Generally, a strict selection is practised
r during inbreeding. Close inbreeding leads to a rapid and random
fixation of genes and does not allow the accumulation of desirable
genes through recombination. It is generally accepted that .selection
accompanied with close inbreeding is more or less ineffective in
increasing the frequency of desirable genes. But selection is highly •
' effective 1 ' for characters with high heritability, and in eliminating
weak and undesirable lines. Thus selection during inbreeding is in-
effective in improving the combining ability of inbreds, but is effective-.
334'
Plant Breeding : Principles and Methods
la improving the performance of inbred s themselves, which is an
important factor in hybrid seed production.
Early Testing. As pointed out earlier, GCA of inbreds can be effec-
tively tested at an early stage during the mbreding programme ; this
is known as early testing. Early testing was proposed by Jenkins
in 1935. In early testing, open-pollinated (So) plants or first or
second generation seifs (Si or S 2 ) may be subjected to the* test for
GCA. The procedure for early testing is simple. First, several So,
S x or S a plants are selected on the basis of phenotype and are self-
pollinated. These plants are simultaneously testcrossed with a tester
with a wide genetic base. The second step consists of planting indi-
vidual plant progenies from the selfed seed and a replicated yield
trial is planted for the testcross progenies. The performance of
testcross progenies is the basis for the identification of superior S 0 , Si
or S 2 plants ; the selfed progenies from such plants are maintained
by self-pollination, while the progenies from inferior plants, are
rejected. The test for GCA may be repeated and used as the basis
for selection of superior or desirable plants in the early inbreeding
generations. There is some evidence that selection during inbreeding
based on the performance of . testcross progeny is highly effective in
improving the GCA of inbreds. Early testing has been widely used in
hybrid programmes, particularly in those species where inbreds tffe
rarely, if ever, developed, e.g in forage crops and clonal crops.
Evaluation of Inbreds
If all the inbreds developed from an open-pollinated variety
were mated at random, the average yield of all the single crosses
would be the same as that of the open-pollinated variety. This is
because it amounts to random mating after several generations of
self-pollination in a Mendelian population. We have already seen
that visual selection during the isolation % of inbreds is not likely to
affect, to any appreciable extent, the performance of hybrids derived
from them. This is self-evident from the fact that of the more than
100,000 inbreds tested till 1950 in U.S.A., only 60 were fit for com-
mercial exploitation. Thus the most important operation in a hybrid
programme is the identification of inbreds that would produce an
outstanding hybrid suitable for commercial use. And undoubtedly
it is the most expensive operation in the development of hybrid
varieties. If n inbreds are to be tested in ail possible single cross
combinations, there would be n (w — 1)/2 single crosses that must be
evaluated in replicated yield trials. With rc=20, the number of single
crosses would be 190. But the number of double crosses from n
inbreds is prohibitively large, that is, 3 Xn\ [(41) (?* — 4!)], which for
20 inbreds would be 14,535. Therefore, double cross combinations
‘of inbreds are not evaluated, except for release as a variety. The
modern practice of inbred evaluation may be divided into the follow-
ing four steps : (1) phenotypic evaluation, (2) top-cross test for
GCA, (3) single cross test for SCA, and (4) prediction of double
across performance from the data on the performance of single
Mpbrtd and Synthetic Varieties
335
posses. These steps, briefly outlined below, are folio wed in the given
Phenotypic Evaluation. It is ' based on the phenotypic performance
of inbreds themselves It ,s highly effective for characters vrithhSb
hen lability high GCA. To some extent, it is effective in mDriv-
ing the yielding ability of hybrids as the yield of inbreds shows a
small (usually 0.4 but positive correlation with the performance of
safelv rejected TlSnSto* wit Vl y ? 0£>r P^manS can be
safety rejected. The performance of inbreds is tested in a replicated
yteld trial, and the inbreds showing poor performance are discarded;
Topcross Test. It is generally accepted that the performance of
topcross progeny of an inbred is a reliable measure of th^eLe
performance of all the single crosses involving an inbred, and K
topcross performance provides a reliable estimate of GCA T1 ■ <*
afte 5 P heQ0{ ypi c evaluation are crossed to a
tester with a wide genetic base e.g., an open-pollinated variety, a
synthetic, or a double cross. A simple way of producing toncross
seed in maize is to plant alternate rows of the tester and the inbreds
to be tested. The inbreds are detasselled ; the seed from the inbreds
is harvested and it represents the topcross seed. The performance
of the topcross progeny is evaluated in replicated yield trials, pre-
ferably over locations and years. Based on the topcross test, about
50 per cent oj the inbreds are eliminated This reduces the number
of inbreds to a manageable size for the next step.
Single Cross Evaluation. It is believed that an important part of
heterosis is due to SCLA. Hence the outstanding single cross combi-
nations can he identified only by testing the performance of single
crosses The final evaluation of the inbreds, therefore, consists of
the evaluation of single crosses produced from them. The inbreds
remaining after the topcross test are generally crossed in a dialled
ma , ni l er ,. to produce all possible- single crosses (reciprocals are not
included). In a diallel system of mating, each inbred is crossed with
every other inbred. This produces n (n-1) single crosses if the
reciprocal crosses are also made, and n (n-1)/ 2 single crosses if the
reciprocals are ignored.
• u P erf ormance of single crosses is evaluated in a replicated
yield trial, preferably over years and locations. Outstanding single
crosses are identified and may be released as hybrid varieties where
production of single crosses is commercially feasible. More com-
monly, e.g. in the case, of maize the performance of single crosses is
used to predict the performance of double crosses.
Prediction of Double Crass Performance. Prediction of double cross
performance from the data on single crosses is a widely accepted
practice. Not only this saves enormous time, labour and money it
is surprisingly accurate. The predicted performance .of any double
cross is the average performance of the four nonparental single crosses
involving the four parental inbreds. Suppose, we wish to predict the
336
Plant Breeding i Principles and Methods
performance cf a double cross involving the four inbreds. A, B, C
g n d D. The six possible single crosses among txiese inbreds would
be A X B, AxC.AxD, BxC.BxD and CxD. These single crosses
can be combined to produce 3 double crosses, viz., (AxB)x(CxD),
(A X C) X (B x D) and (A X D) X (B x C). The performance of any of
these double crosses can be predicted from the performance of the
four single crosses net involved in producing that particular double
cross. For example, the performance of the double cross (AxB)x
( r X D) would be the average of the performance of the single crosses
AXC AxD BxC and 3xD, since these single crosses are not
involved in producing this double cross. Similarly, the performance
of the double cross (AxC)x(Bx D) can be predicted from the
average of the performances of the single crosses A X B, A X D,
BxC and CxD. Which single crosses would be used to predict the
performance of the double cross (A X D) X (B X C) ?
The order of inbreds in a double cross is decided by their genetic
relationship. Inbreds that are genetically related, *.<?., derived from
the same or related sources, are used to produce the single crosses.
Geneticallv unrelated single crosses are then mated to produce the
double cross. For example, if inbreds A and B are from one source,
while C and D are from another, A would be crossed with B, and C
would be mated with D to produce the single crosses A x B and
CxD These single crosses would then be mated to produce the'
double cross (AxB)x (CxD) ; this would be the best double cross
from these four inbreds.
Production of Hybrid Seed
The two requirements of commercial hybrid seed production-
are : ( 1 ) easy emasculation of the female parent, and (2) effective
nollen dispersal from the male parent to, ensure a satisfactory seed
set in the female parent. Both these factors are largely governed by
the floral structure and the natural mode of reproduction of the crop
species in question. For example, emasculation m maize, due to its
peculiarly favourable floral structure, consists of a very simple opera-
tion of detaselling, i.e., removal of the entire tassel (the male
inflorescence) from the plant before pollen is shed. Thus one simple
operation emasculates the whole plant. The production of hybrid
maize till 1960s was essentially based on detasselhng. However,
male sterility and self-incompatibility offer the means fox genetic
emasculation, that is. preventing self-fertilization by manipulating the
genotvpe of the plant, and are the basis of hybrid seed production m
many crops. Pollen dispersal is often satisfactory m most cross-
pollinated species since it is their natural mode of reproduction. But
in self- pollinated species, satisfactory pollen dispersal is often the
limiting factor in hybrid seed production. Hybrid seed may be piO~
duced in one of the following several ways : (1) cytoplasmic-genetic
male sterility, (2) cytoplasmic male sterility, (3) genetic male sterility,
(4) self-incompatibility, and (5) manual emasculation and/or pollina-
tion, The first four systems have been described in some detail m.
Hybrid and Synthetic Varieties
337
Chapter 3 where their merits and demerits have -been discussed. The
first and the fifth system have been described in Chapter 16 in rela-
tion to jo war and cotton, respectively.
Cytoplasmsc-Geneflc Male Sterility. This system is the most widely
used method in hybrid seed production. It Is commercially used in
maize, bajra, jowar, onions (A. cepa) and sugarbeets (B, vulgaris).
The system is based on a cytoplasm that produces male sterility, and
on a gene that restores fertility in the presence of the male sterile
cytoplasm. The development of male sterile and restorer lines
has been described in Chapter 3. ' The use of this system in hybrid
seed production is outlined below.
Production of Single Cross Hybrid Varieties. For’ the production of
a single cross, a male sterile line is used as female, and the male
parent is a restorer. The seed set on the female parent (the male
sterile Hoe) is the hybrid seed, while that produced on the male
parent is selfed seed. The resulting hybrid is male fertile since it has
.received the restorer gene from the male parent (Fig. 19.2). Gene-
rally, two rows of the male ferule inbred (the male parent) are
planted after every two rows of the male sterile parent (the female
parent). ■ But when the male inbred produces sufficient pollen, 2 rows
of the male inbred may be planted after every 3 or 4 rows of the
female parent. The present single cross hybrid varieties of maize
are produced by planting male and female inbreds in the ratio
of 2 : 4.
Production of Double Cross Hybrid Varieties. Double cross hybrid
varieties are produced by crossing two single crosses, one male sterile
and the other male fertile. The male sterile single cross is produced
by crossing a cytoplasmic male sterile line with a- non-restorer male
fertile line [Fig. 19.2, Single Cross (A X B) of the Double Cross
Schemes I and II]. The male fertile single cross may be produced in
one of die two ways. First, a cytoplasmic male sterile line is crossed
with a restorer line (Fig. 19.2, Scheme!); the double cross in this case
has both male sterile and male fertile plants in the ratio 1:1. In the
second method, two restorer lines are crossed together ; one of the
restorer lines serves as female and is detasselled manually (Fig. 19.2,
•Scheme II). All the plants in double cross would be male fertile.
Scheme II is expensive, but It may have some usefulness in crop
species where pollen production is limited. Even In maize, in certain
areas pollen production may be poor making the Scheme II
desirable.
Cytoplasmic Male Sterility. The scheme for hybrid seed production
is the same as that with cytoplasmic- genetic male sterility, except
that the male fertile line is nonrestorer (by the definition of cytoplas*
mic male sterility system). The hybrid, therefore, is male sterile.
It may be useful in crops where grain or seed is not the commercial
product. It has not been exploited commercially to any appreciable
extent.
Genetic Male Sterility. The male sterile line (ms ms) is allowed to be
cross-pollinated with a male fertile line (Mr Ms) to vield a
338
Plant Breeding : Principles and Methods
»mmo A
tj> CVTOPLASM/C
MALE STERILE ,
6
INBRED B
i RESTORER ,RRj‘
SfMGLf CROSS
wmmmAKB
4S£ED HARVESTS© MSVICO
A i ALL PLANTS Mate FIHTK.8., «*i
INBRED A
hCYTOPLASMiC
MALE STERILE
fNBRED B
(NON RESTORER
INBRED C
J CYTOPLASMIC
MALE STERILE
4
INBRED D
{RESTORER i HR)
DOUBLE CROSS
SCHEME T
SINGLE CROSS
eK X S
SINGLE CROSS
C*Q
( MALE- FERTM-6 ,
Rrji
DOUBLE CROSS
i A x B ) x { C x D >
n FERTILE, Rr;
1 STERILE j ft)
>iBREO A
(CYTOPLASMIC
MALE STERILE
tS
iN'BRED B
{NONRESTORER
<NBR£0 C DE TASSBL
(RESTORER rRB,
iNQRED O
i RESTORER »RR)'
SINGLE CROSS
Ax B
(Male sterile
r n
SINGLE CROSS
CxD
(MALE FERTILE
RR)
DOUBLE CROSS'
(A xB) x (Ox Dj
(ALL PLANTS
MALE FERTILE
Rn
Fig. 19,2. The use of cytoplasmic-genetic male sterility 'in the production of
hybrid seed.
fertile hybrid (Ms ms). The seed produced 00 male sterile line is the
hybrid seed. The development and maintenance of male sterile lines
has been discussed in some detail in Chapters., It has been
commercially exploited in castor vU.S.A.) and pigeonpea (India, m
a small scale). -
Self-Incompatibility. Two self-incompatible but cross-compatible
lines are planted in alternate rows; the seed produced by both the
lines would be hybrid seed. Alternatively, a self-compatible line
may be interplanted with a self-incompatible line. In this case, the
■seed front self-incompatible line will be the hybrid seed, while that
Hybrid and Synthetic Varieties
339
from the self-compatible line will be a mixture of hybrid and selfed
^ seed . Therefore, the seed from self-incompatible line only is used as
the hybrid variety. This system is being commercially used for
hybrid seed production in some Brassica crops in Europe and Japan,
e.g,, Brussel’s sprouts (5. Oleracea) and cabbage (B. oleracea).
Manual Emasculation and/or Pollination. This method relies on
manual emasculation and, in many cases, on manual pollination.
Early hybrid maize production was based on manual emasculation,
i.e., detasselling. The Scheme II of double cross production
(Fig. 19.2) also involves detasselling for producing the single cross
(CxD). Manual emasculation and pollination has been successfully
used for hybrid jseed production of tomatoes in Europe, and of
cotton in India. The hybrid seed produced by manual operations is
, * very costly. Consequently, this system of hybrid seed production is
limited only to those crops where the returns are very high making
the production of hybrid seed economically feasible.
Improving The Characteristics of Inbred Lines
The direct isolation of inbreds from source populations and their
evaluation is a time consuming and expensive process. Further, the
frequency cf outstanding inbreds in direct isolations is very low. As
a result, various schemes have been suggested to improve the existing
inbreds in respect of (1) the productivity of inbreds to make them
suitable for use in hybrid seed production, (2) disease and insect
resistance or some other characteristics of the inbreds so that the
characteristics of the hybrid are improved, or (3) the combining
ability of inbreds to increase the yielding ability of their hybrids.
f One or more of these objectives are fulfilled by the following
methods : (1) pedigree selection, (2) baekcross method, (3) conver-
gent improvement, and (4) gamete selection.
Pedigree Selection. Pedigree selection consists of the isolation of
inbreds from an outstanding single, or even a double cross. Two
inbreds that complement each other ‘for disease resistance and for
other desirable attributes are crossed and in the segregating genera-
tions desirable recombinants are selected. The segregating genera-
tions are produced by selling or close inbreeding. The method is
closely similar to the pedigree method used in self-pollinated crops,
as is suggested by its name. The inbreds produced in this manner
are known as second cycle inbreds. Second cycle inbreds are consi-
derably superior to the first cycle ones -in-their per se performance.
The performance of their hybrids may also be improved, but
generally the improvement is not substantial.
Baekcross Method. This is essentially the same method as that
applicable to the self-pollinated crops (Chapter 15), and has the same
applications. An otherwise desirable inbred is used as the recurrent
parent in a baekcross programme. The donor parent is an inbred
;that has the desirable characters in a high intensity. Baekcross
S40
Plant Breeding : Principles and Methods
tfettiod cari'be used to transfer any character that has high herita-
.biiity. It has been used to improve disease resistance, insect
resistance, e.g., resistance to stem border in maize, lodging resistance-
arid several other characteristics ofinbreds. . A specialised applica-
tion of- the'backcross method is the transfer of cytoplasmic male
sterility and of restorer genes to new inbreds. An example of the-
.improvement of inbreds through the backcross method is furnished
by bajra. The male sterile line Tift 23 A, introduced from U S. A,,
highly susceptible to downy mildew leading to the. high suscepti-
bility pf the 'hybrids produced by using it as the female parent.
‘Downy mildew resistant male sterile lines have been produced by
backcrossing Tift 23A to some downy mildew resistant African and
Indian lines.
emergent Improvement, It is a special case of backcross ; a single-
cross is backcrossed separately to the two parental inbreds . Selection
is made on the basis of phenotype during the backcrossing. It is-
hoped that the two parental inbreds, e.g,, A and B, would be
improved by retaining some of the -favourable genes contributed by
the other unbred. Thus A is expected to be improved by the genes
from B s and B is expected to be improved bv the genes from A.
There is- some evidence that this method improves, to some extent,
the performance ofinbreds as well as that of the hybrids produced
from them. But generally the improvement is not substantial, and the-
‘method has not been widely used.
Gamete Selection. Gamete selection was proposed by Stadler in-
1944. The basis for this scheme is the consideration that the fre-
quency of superior gametes (p) in a random mating population is
appreciably higher than that of the superior zygotes (p% This is*
particularly so when p< 0.5.
In gamete selection , a good inbred line is crossed with a random
sample of gametes from an open-pollinated variety. The resulting;
Fi plants- are selfed as well’ as crossed to a suitable tester. The tester
may be an inbred or a population with a wide genetic base, depend-
ing upon the objective of the programme. The testcross progeny
. are evaluated in a replicated trial The differences in the performance
of the testcross progenies would be entirely due to the gametes from
the open-pollinated variety, since the other parent is an inbred which
would be homozygous for all practical purposes. Selfed seeds from the
Fi plants that- produced superior testcross progenies are ' planted in
progeny rows (Fig, 19.3). Selling and selection is continued to
develop inbred lines which are expected to be superior to the inbred .
used as one of the parents,
This scheme has not been used on a large scale for the develop-
ment of inbred lines. It is argued that the scheme has considerable-
■potehtial and should be widely used for producing superior inbreds.
(0 Inbred:
sibrnating.
Hybrid and Synthetic Varieties
*Wf
VC
»m*f o a
/
OPEN ■ P Oil tN A 7£ O
VArtiCTv '
ttecoNQ
VC AR
jOOOGOO
joo OOOO
jO GOO Do
p o O o o O
j o OOO O Q
f 5 POPU« ATiOMl
OOOOC
fOCOGO
fOGOOO 5
ooooo
|
■ 1
TEST-CROGI PROGer.M
rn<RO
v EAR
rTrnTi.ii.nMi
ii Mill: Mini
| H II | If 1 iti 1 1 1
j
Bii PLICA TE D
1
!
fourth
TEAR
f
imnimiiiiii
FIFTH TO
FSGHTH
tear
NfNTH OB
tenth
VEAR
SFlFCp PROGS: NiCS
SEt F tKiO,
UMi I ill 1 1 j Iff
SGlFED PROGENIES
A gond # inbred A is polli-
nated with a random sam-
PiC of pollen from an open-
pollinated, variety.
lj/ p i is space-planted*
00 Fi plants are test-cross-
self* pollinated.
(lnjSejfed seed is kept for
, use Jn the fourth year,,
L‘ u f. d W Testcross progenies are
ur© planted in a replicated trial
00 Superior Fi plants are
identified on the basis of
performance of J their test-
cross progeny.
0) Selfed seed from the Fi
plants that produced sooe-
rior testcross progenies are
planted in progeny rows.
(ii) Superior plants are
selected and selfed.
0) Selfed seed from the
.selected plants sown : in pro- ■' r
geny rows.
(ii) Superior plants are
selected and selfed.
maintained by
Fig. 19.3. Gamete selection in maize for the improvement of inbred lines.
Merifs of Hybrid Varieties
Hybrid varieties exploit both GCA and SCA. component
extent" 03 ' 5 ' ^ th * y U£ ' IiZS hsterosis t0 «w greatest possible
The produce from hybrid, particularly single cross varieties
is more uniform as compared to that' from open pollinated
synthetic or composite varieties. P P ted ’
Scies an TLf r 2 dUC S botJ ? in cr °ss- and self-pollinated crop
species. I hey are the only possible means of exploiting
842
Plant Breeding : Principles and Methods
heterosis in self-pollinated species, as open-polfinated, synthetic
and composite varieties are not possible in them,
4* Hybrid varieties are maintained in the form of their parental
inbreds, which are grown in isolation and subjected to sib-
mating. This ensures that the genetic constitution of a hybrid
variety does sot change with time, except from some spontane- '
ous mutations. In contrast, the genetic compositions of open-
pollinated, synthetic and composite varieties are likely to change
with time due to natural and unintended artificial selection.
5. In many self-pollinated crops, hybrid varieties yield 25-30%
more than the pureilne varieties. As a result, they are becoming
popular among the farmers despite the high -cost of hybrid
.seed (e.g., Rs. 50,00 per kg. for hybrid arhar and upto
Rs. 150.00 per kg. for hybrid cotton).
B&mef&is of Hybrid Varieties
h Farmers have to use new hybrid seed every year.- They cannot
produce their own seed.
2 . . Hybrid seed production requires considerable technical skill.
This makes hybrid seed production a tedius and costly affair.
3. The exploitation of full potential -of hybrid varieties requires
an adequate supply of irrigation water and fertilizer, and
control of weeds, diseases and insect pests. Many farmers
are unable to ensure a timely application of these essential
inputs.
4. The large scale production of hybrid seed depends on easy
emasculation of the female parent, and on an adequate pollen
dispersal from the male parent. In many species, emasculation
and pollen dispersal are either unsatisfactory or not practical!;
feasible. In such species, e.g., in most self pollinated crops
hybrid varieties cannot be produced on a commercial scale
. unless their floral biology is adequately modified through breed*
mg/genetic manipulations.
5. In mo*t cross-pollinated species, the requirements of isolation
are rigid and, -ordinarily,' difficult to fulfil, except on large
farms. This renders hybrid seed production rather difficult
in a country like India where farm holdings are generally
small.
6. The amount of hybrid seed required to cover the entire 'area
under any crop' appears to be impossible to produce with our
present seed ^production setup. At present,, the total certified
seed production in the country is just enough to cover only
about 4.6% of the total seed requirement 'of all the crops
{including self-pollinated crops, where seed production is
relatively much easier). ' ■
Achievements through Hybrid Varieties
Hybrid varieties have been commercially exploited in most of
the cross-pollinated crops. The greatest use of hybrid varieties has
Hvbrid and Synthetic Varieties
been made in maize. Other cross-pollinated crop species where
hybrid varieties have been commercially exploited are jowar, bajra,
sugarbeets (B. vulgaris X sunflower (H. annum'), broad bean (Vida
fata), onions (A. cepa ), etc. Hybrid varieties have been successful in
some seif-pollinated crops as well, e.g., cotton (Gossypium sp .) in
India* tomatoes in Europe and rice in China. Most of the self-polli-
nated crops .do show . heterosis, but hybrid varieties are not feasible
due to the difficulties in hybrid seed production.
Hybrid- varieties were the first to exploit heterosis in maize in
this country. The first hybrid varieties in maize were released in
1961 when four hybrids, viz., Ganga], Ganga 101, Ranjit and
Deccan, were released for cultivation. All these hybrids are double
crosses. Subsequently, several other hybrids were released. At
present, about a dozen double cross hybrid varieties are recommend-
ed for cultivation. These hybrids include Ganga 3, Ganga 4, Ganga
5, Ganga 7, Deccan 101, Bi-Starch and Ganga Safed 2. These
hybrids, give 25-40 per cent higher yields than the local opea-polli-
Bated varieties.' The hybrids have yielded up to 4 tonnes per hectare
of grain oo the fanner’s fields.
Hybrid varieties have been commercially used in bajra. The first
hybrid bajra was developed by the Punjab Agriculture Uaiverslty,
344 Plant Breeding t Principles and M&tktfds
Ludhiana, and was released ia 1965 as HB 1. Subsequently, two
othir hybrids, HB 3 and HB 5, were released. These hybrids gave
yields up to 7 tonnes per hectare in the National Demonstration
Trials. However, these hybrids were highly susceptible to downy
mildew and ergot due to the high susceptibility of the female parent
Tift 23A. These bajra hybrids are single crosses produced by using
cytoplasmic-genetic male sterility ; Tift 23A introduced from Tifton,
Georgia (U.S.A.), was used as the female parent.
Backcrossing and mutation breeding was used to develop
several male sterile lines of bajra resistant to downy mildew and
ergot. These male sterile lines are MS 521, MS o4!A, MS 57GA,.
MS 5071 and L ill A. Four hybrid bajra varieties based on the new
mate sterile lines have been released for cultivation. These hybrids
are, PHB 10, PHB 11, BJ 104 and BK 560, which are resistant to
downy mildew and are recommended for general cultivation in the
bajra growing areas. These hybrids are capable of yields up to
6 tonnes per hectare under optimum management.
Hybrid varieties have been released for commercial cultivation
in jowar and cotton (Chapter 16). Hybrid vigour has been exploited
in many clonal crops and in several fruit trees. An excellent example
is provided by coconut (Cocos nuciferd). the open-pollinated
variety West Coast Tall yields upto MO nuts per plant per year
Fig. 19.5. A close view of a hybrid bajra (Pennisetum americanun) crop( HB-4)
Hybrid and- Synthetic Varieties
345
under optimum management. The hybrid between West Coast Tall
and Chowghat Dwarf Orange (T? x D f for short) yields upto 175
nuts per plant, while the -reciprocal hybrid (D? xT-J) yields upto
200 nuts per plant. The coconut example also provides evidence for
cytoplarniic effect on hybrid vigour (compare the performances of
the TX D and Ox T hybrids). In addition to being ' higher \ieldiri.fc
the TxD and DxT hybrids are earlier in first flowering (earlier by
about 10*1 ! months) than the parents. w ’
Synthetic Varieties
The possibility ot commercial utilization of synthetic varieties
in maize was first suggested by Hayes and Garber in 1919. Synthetic
varieties have been of great value in the breeding of those cross- polli-
nated crops where pollination control is difficult, e.g , forage crop
species, many clonal crops, like xacao, alfalfa (M. sativa), "clovers
(TrifouHm sp.) etc. Even in maize improvement, synthetic varieties
are becoming increasingly important. The maize improvement pro-,
gramme in India now places a considerable emphasis on synthetic
varieties. The maize programme of CIMMYT, Mexico, is based on
population improvement ; the end-product of such a programme is
usually a synthetic variety.' The same applies, to a lesser extent, to
the near! millet (P, americamm) improvement programme of
ICRISAT, India.
Definitions
A synthetic variety is produced by crossing in all combination* "'a
number of lines that combine well with each other.. Once synthesized ,,
a synthetic is maintained by open-pollination in isolation. Some
breeders use the term synthetic variety in a restricted sense : a
synthetic variety is regularly reconstructed from the parental fires
and is not maintained by open-pollination. In this chapter, however,
the term is used to describe varieties -maintained by. r open-pollination
after their production from a number of lines.
Another term, composite , is often used as a synonym .for
•synthetic, which k hot entirely accurate. A composite variety k pro-
duced by mixing the seeds of ..several phjenotypi'caljy outstanding-lines
.and encouraging open-pollination to produce crosses in ail combina-
tions among the mixed lines. The lines used to produce a composite
variety are rarely tested for combining ability with each other.
Consequently, the yields of composite varieties cannot be predicted
■i n advance (in constrast to synthetics) for the obvious reason that
the yields of a 1 ! the FjS among the component lines are not available.
Like synthetics, composites are commercial varieties and are main-
tained by open-pollination m isolation.
Germplasm complexes are produced by mixing seeds from several
lines or populations of diverse genetic . origin. The objective of
.germplasm complexes is to serve as reservoirs of germplasm.' Germ-
plasm complexes are. experimental populations and not commercial
'varieties.
346
Plant Breeding l Principles and Methods
Operations in Producing A Synthetic Variety
By definition, a synthetic variety consists of all possible crosses-
among a number of lines that combine well with each other. The
lines that makeup a synthetic variety may be inbred lines, clones,
open-pollinated varieties , short-term inbred lines or other populations
tested for GCA or for combining ability with # each other . The opera-
tions involved in the production of synthetic varieties sue depicted,
in Fig. 19.6 and are briefly described below.
Evaluation of Lines f or GCA. GCA of the lines to be used as the
parents of synthetic varieties is generally estimated by topcross or
polvcross test, polycross refers to the progeny of a line produced
bv pollination (usually natural pollination) with a random sample
of pollen from a number of selected lines. Polycross test is the
most commonly used test in forage crops. Pclycross progeny are
generally produced by open-pollination in isolation among the selec-
ted lines. The lines are evaluated for GCA because synthetic varie-
ties exploit that portion of heterosis which is produced by GCA,
The lines that have high GCA are selected as parents of a synthetic
variety.
Production of A Synthetic Variety. A 'synthetic variety may be pro-
duced in one of the following two ways :
L Equal amounts of seeds from the parental lines (Syno) are mixed
and planted in isolation. Open-pollination is allowed and is
expected to produce crosses in all combinations. The seed from
this population is harvested in bulk the population raised
from this seed is the Syn 2 generation,
2 n All possible crosses among the selected, lines are made in isola-
tion. (The parental lines constitute the Syn© generation.) Equal
amounts of seed from each cross is composited to produce the
synthetic variety. The- population derived from this composited
seed is known as the Sym generation.
The available experimental evidence suggests that both the
above methods produce comparable results.
Multiplication of Synthetic Varieties. After a synthetic variety has
been synthesised, it is generally multiplied in isolation for one or
more generations before its distribution Tor cultivation. This is done
to produce commercial quantities of seed, and is a common practice
in most of the crops s e.g. 9 grasses, clovers (Trifolium sp\ maize
etc. But m some crops, e>g. 9 sugarbeets (B. vulgaris X the synthetic-
varieties are distributed without /seed increase, i.e. 9 in the Syii|
generation.
The open-pollinated progeny from the Syn% generation is term-
ed as Syn 2 , that from Syc 2 as Syn B etc. The performance of -Syn* is.
expected to be lower than that of Syn’i'due to the production of new
genotypes and a decrease in heterozygosity as a consequence of
random mating. However, there would not be a noticeable decline in
the .subsequent generations produced by open-pollination (Syn 3s Syn 4 ».
Syns etc,) since the zygotic equilibrium for any gene is reached after
STEP i.
.(VALUATION
Of LINES
POS{ ©ca
lopcioss of polycross
test for GCA; outstaod*-
ing lines selected as
parents
’ Short -ter
JNBRgQS,
„ TESTER
(OPEN-POLLINATEO*
VAftlETYj
Method I. Equal seed
from all the lines mixed
and planted in Isolation.
Open-pollinated seed
Harvested as the syn-
thetic variety (Syn 2 ).
METHOD I, COMPOSITE
OF ALL LINES
Method 2. The parental
lines are planted in a
crossing block. All
possible intercrosses are-
made. Equal seed from,
all the crosses mixed to-
produce the synthetic
va rkty (Syn $ ).
OPEN 'POLLINATED
SEED HARVESTED
METHOD II.CROSSING ©LOC*
EQUAL SEED
FROM ALL CROSSES
COMPOSITED
STEPS, see
MULT-' PLICA*
Seed f of the syi
variety^may be"
plied for.. one .01
generations befos
tri button. Open
nation . in iso
($yn a or Syn 3 ),
sc varieties.
SEED MULTIPLICATION •
; {OPEN-POLLINATION !JM
ISOLATION) '
Fig. 19.6. Steps Involved in the production ofsynthetii
foils of Synthetic Varieties
34?
Plant Breeding : Principled and Methods
2. Tfee farmer can use the grain produced from a synthetic
variety as seed to raise the next crop. If care is -takes to avoid
contamination by foreign pollen, and to select a sufficiently
large number of plants to avoid inbreeding* the' synthetics can
be maintained for several years from open-pollinated seed.
Unlike hybrid varieties, the farmer does not have to purchase
new seed every year-
3. In variable environments, synthetics are likely to do better
than hybrid varieties. This is so because of the wider genetic
base of synthetic varieties in comparison to that of hybrid
varieties.
4. 1 fee cost of seed in the case of synthetic varieties is relatively
lower than that of hybrid varieties. Thus in a country like
India, where most of - the farmers have limited .financial
resources, synthetic varieties are more attractive than hybrid
varieties,
5. Seed production of hybrid varieties is a more skilled operation
than that of synthetic varieties.
6. Synthetic varieties are good reservoirs of genetic variability.
The composites and geftn'plasin complexes also serve as gene
reservoirs. Germplasm complexes are, in fact, great reservoirs
of genes created by mao for this purpose alone.
7. There is good evidence that the performance of , synthetic
varieties can be considerably improved through population
improvement without appreciably reducing variability. This
offers a possibility for a continuous improvement, which is not
possible with hybrid varieties.
Demerits of Synthetic Varieties .
1, The performance of synthetic varieties is usually lower than
that of the s ingle or double cross hybrids. This is because
synthetics exploit only GCA, while the hybrid varieties exploit
both GCA and SCA.
2. The performance of synthetics is adversely affected by lines
with relatively poorer GCA. Such' lines often have to be ■ inclu-
ded to increase the number of lines making up the synthetic as
lines with outstanding GCA are limited in number.
. 3. Synthetics can be produced and maintained only in cross-
pollinated crop species, while hybrid varieties can be produced
both in self and cross-pollinated crops.
Factors Determining The Performance of Synthetic Varieties
The yield of Syn 2 , as noted earlier, is lower than that of Syni
due chiefly to production of new gene, combinations an<J, to some
extent, a loss' in heterozygosity, both being the consequences of
random mating in- Syn 1# Random mating in Synn leads to a
marginal to appreciable loss in heterozygosity in Syn 2 as compared
to that', in Syn*. The magnitude of decline in heterozygosity in
Inbred genotype combination
Generation
Synx
Syria
Decline In beterozy-
gosityin-Syna
Combination
Inbred
I
2
3
4
5
6
I
M
AA
A A
AA
- AA
AA
II
AA
AA
AA
AA
AA
aa
ill
A A
AA
AA
AA
m
aa ■ .
IV
AA
AA
AA
aa
aa
aa
V ' "
AA
AA
aa
aa
aa
aa
VI
AA
aa
aa
; ■ aa
aa
aa
VII
aa
aa
aa
m
aa
aa ■
Table 19.2, ■ Heterozygosity (%) in the Syni and Syng generations of a synthetic
constructed from the sis inbreds listed in Table 19.!
350
Plant Breeding i Principles and Methods
The yield of Syn«, as noted earlier, is Sower than that of Sym
mainly due to the loss in heterozygosity as a result of random gating.
The decrease in yielding ability of Syn 2 generation would depend
upon (1) the number of lines (Syn, populations) constituting the
synthetic, and (2) on the difference in the yielding abilities cr Syn*
and Svno generations. Sym is the first generation syntnetic produced
by mating in all combinations n parental lines (designated as Sym).
This relationship was first suggested by Sewall Vnght in 19_2 and
may be represented as follows.
Syn»=Sym— [(Sym — Syno) In ]
where, Syn 2 is the performanc e of the synthetic variety after one
generation "of random mating, Syn 2 is the performance of the syn-
thetic in the first generation after it has been synthesised from the
parental lines (it could be the avera ge performance of the all possible
single crosses among n lines), Syn 0 is the average performance of the
n parental lines, and n is the number of lines entering into the syn-
thetic variety.
The performance of Syn 3 and the subsequent generations
obtained by random mating is expected to be comparable to that of
Syn 2 , and there should be no further decline. Available evidence
shows that the above formula estimates the yield of Syn 2 populations
quite reliably. It is apparent, from this relationship that the perfor-
mance of Syn 2 can be improved in 3 different ways : (1) by
increasing the "number of lines entering into the synthetic, (2) by
increasing the performance of Sym, and (3) by improving the
performence of Syno, or the parental lines.
increasing The Number of Parental Lines. It is clear from the above
formula that, theoretically, an increase in the number of lines enter-
ing a synthetic would improve the performance of its Syn 2 . This
improvement would be more noticeable with the smaller values of
n, e g., 1/2, 1/3, 1/4, 1/5 etc., but would become less noticeable as
the n becomes larger, e.g., 1/10, 1/11, 1/12 etc.
Practically, lines with outstanding GCA are few. Inevitably,
as the n is increased, lines with poorer GCA would have to be
included in the synthetic. This reduces the performance of Sym and,
therefore, that of Syn 2 . Thus n cannot be increased beyond a certain
level without adversely affecting the performance of Sym unless
several lines with outstanding GCA are available. Generally, a
compromise has to be made between the two opposing forces.
Obviously, the appropriate n would depend upon the GCA of the
available lines. If several lines with high GCA are available, a larger
n would be appropriate than when a fewer lines with high GCA are
available.
In practice, the number of lines entering a synthetic variety
varies from 3-15, but 4-1 0 is the most common number. Some of the
maize composites recommended for commercial cultivation in India
have as many as 18 (Composite Cj) or even 22 (Composite Agaiti 76'
Hybrid and Synthetic Varieties
351
f
, ).
f
' £
i
parental lines. In contrast, some other composites are crosses bet*
ween two populations of maize, e.g., composite Tarun is derived
from Syn P200x.Kisan, or are derived from a single population,
e.g.. Chandan Safed composite is an improved version of Zapolote
Chico, an exotic variety, and Makki Safed 1 composite is derived
from Bhodipur white open-pollinated variety of maize.
Increasing The Performance of Syn,. The performance of S,vn s is the
average performance of all single crosses among the parental lines
Clearly, Syn, performance depends upon GCA of the parental lines.
As noted previously, the Syn, performance is limited by the oon-
avilabihty of a sufficient number of lines with outstanding GCA.
At the same time, if the number of lines is kept too low, say, 3
and 4, Sym performance would be high but the Syn*, performance
would be adversely affected due to the low number of “parental lines.
Improving The Performance of Parental Lines (Syn 0 ). Syn 2 perfor-
mance can be improved by increasing the performance of parental
lines. This can be achieved in the following manner. First," second
cycle and third cycle inbreds may be developed as their performance
would be better than that of the first cycle inbreds. Second, inbreds
may be isolated after'the population has been subjected to recurrent
selection for GCA. And third, short-term inbred lines or even noa-
inbred lines or populations may be used for the production of synthe-
tic varieties.
The use of short-term inbreds is an attractive idea. Plants differ
in GCA in the early stages of inbreeding process ; even S 0 plants
differ in GCA. Thus the synthetic varieties may be constituted from
plants or lines that have undergone limited inbreeding, e.g., for one
or two generations, or no inbreeding at all. Theoretically,' the per-
formance of a Synj from non-inbred lines is expected to be higher
than that of a Syn 2 derived from inbred lines if the performance of
the Syn, populations is comparable or equal. Further, the peak
performance of Syn 2 would be attained with a relatively smaller
number of noninbred lines than with inbred lines. It may be pointed
out that sythetics constituted by crossing of populations subjected to
RSGCA and RRS would serve the same purpose as developing
synthetic varieties from short-term inbreds or non-inbreds.
Maintenance of Synthetic Varieties
Synthetics are generally maintained from open-pollinated seed
produced in isolation. The seed preserved for raising the next gene-
ration is a random sample large enough to prevent inbreeding. How-
ever, there is evidence that the genetic constitution of synthetic
varieties changes due to natural and artificial selections. This may
adversely affect the performance of synthetic varieties, but in some
cases it may improve the performance appereciably. Hence it is desir-
able to reconstitute synthetic varieties at regular intervals from the
parental lines. Exact reconstitution of a synthetic variety is possible
when the parental lines are inbreds or clones. But when the parental
352
Plant Breeding : Principles and Methods
! lines are noninbred or open-pollinated populations, an exact re-
,/ ] ; constitution, is not possible since the parental lines themselves are
subject to changes io gene frequencies due to the evolutionary forces.
In such cases, the synthetic varieties have to be maintained from
open-pollinated seed, or have to be reconstituted from the parental
h ' lines which might have undergone genetic changes.
The synthetic varieties have considerable genetic variability
: 1 which responds to population improvement. They offer an excellent
opportunity for further improvement in yielding ability through
selection, particularly recurrent selection. If care is taken to minimise
inbreeding, selection may be expected to produce gains through
Jj several cycles of selection.
Test for Combining Ability in Species Where Pollination Control Is
| • ■ Difficult
Synthetic varieties offer a feasible means for exploiting a subs-
tantial portion of heterosis in crop species where pollination control
,'{ ; ' is difficult. In such species, usually noninbred populations or clones
I (where possible) are used for the production of synthetic varieties.-
The clones or populations that enter in a synthetic must be evafuted
for GCA. The technique for testing GCA in case- of maize, where
pollination control is easy, was considered in connection with hybrid
varieties, and is based on topcross progeny test. In this case, the
topcross seed results from inbred x open-pollinated variety cross.
But in species where pollination control is difficult, topcross seed
also includes seed from selling (except where the clones are self-
incompatible) and from intercrossing with other selected clones or
populations. Consequently, several techniques for testing GCA have
been developed for use in such species.
Open-Pollinated Progeny Test, Open-pollinated progenies are derived
from seed produced by random mating of selected plants or clones
with other plants or clones present in the nursery. Random mating
takes place by the natural forces that favour cross»pollicationin the
species in question, and no pollination control is exercised. Thus
open-pollinated seed also contains some selfed seed. Open-polli-
nated progeny may be similar to polycross progeny when only the
selected clones are planted in the crossing block.
Topcross Test. Topcross seed is obtained by crossing- the selected
clones with an open-pollinated variety. In practice, it also contains
selfed seed and seed produced by outcrossing with the other selected-
clones. The proportion of topcross seed can be increased by
planting a greater number of plants of the tester strain.
Polycross Test. Polycross test is based on the seed obtained by ran-
dom mating of a selected clone with all the other selected clones. To
facilitate random meeting among the clones, each clone is planted at.
several locations in the nursery, preferably at different dates. Poly-
crosses are generally not perfect, i.e , 9 mating is nonrandom, and in»
many cases the mating may be highly nonrandom.
ijt.
Sfybi-id and Synthetic Varieties
353
^iS^SSySTTSr^^ asss 1 * is:
or sca a, ^ ^itsisr^s
matit,g n G P CA Ct in e such Sop Sjf ^TheT C ,°T 0n ^ USed for esti '
that it is a more reliable testforOCA tLi’Si?* e » deDC !; md,cates
test or top-cross test. A th °pen-polhnated progeny
Achievements through Synthetic Varieties
, Synthetic varieties have been widefv uc**ft in #*„,.« » «
in crops where pollination control is difficult Thp 3 c f°P s and
programme at CIMMYT mL , The maize breeding
SFESafiSS3i£e=
isr cr emphasis °° ,be ?ro3 “ ti » -%y?s%53s
.. -* n * nd * a ’ *^ e hrst composite varieties were released in 1967 ■
been released bv bnth^t^f n ^ nambe f of other composites have
SmSS It ^reS? ,he« a“ abou.ls ST”™ 1 ,ari . ety
mended for colti ration in the S»°pa« s ST recom -
posttes range from those based on local 'materials (MaS SafS”t
(M.n“? ) 'ch h a!; ,h , 0 S ^ V 0 , r S bMh “a„d <M ef„ric “fee's
frbm Sot if ite? i 3, Oiara Composite etc.) to those developed
etc? Some ofther ™ n (CI ? t aridan f afed 2,' Amber Pop, Chandan 1
or even 22 f Asad v TfiE T, O VG as - maay as 18 (composite C t >
n^hnr3- 8 t c 76 K hnes ’ whlIe certain ot hers involve only two
population^ ’ ° Da) ° f CVCn ° Ee (Chandan Safed 2 > A-de Cuba)
K «.c«5° me - +** the re ? ent, y released maize composites are : Co I (Full
R?nSk?ver?r I 0 | d r rty rf i3d r ew) ’ NLD Reason, whi to seeded)
Renuka (very early), Kanchan (very early), and Diara 3 (develooed
by three cycles of full-sib selection in Diara Composite) P
hybd^tTrietiS^W £ e!d f m<Ich . as 90 per cent of the best
Pmtina havS b^JJli P P%1 composites, v/z.Shakti, Ratan and
and dvritnrihhn • J hey \ ave twice the amount of lysine
comSe° s Ph T h aS f? mpared . t0 , the normal maize hybrids and
sdSr th ,'h^L h DUt . rition al value of these composites is
superior to the other composites and hybrids.
— - i
354 Plant Breeding f Principles and Methods
SUMMARY
The two methods of breeding thM utilize
synthetic varieties. Hybrid ‘ . Commercial hybrid varieties
between two clones, inbreds o _ other P°P u,a ^ n5 ,- ’ ^ hybrid varieties are
are either single crosses maize (in U.S.A). Double
c»mmon in bajra,jOWM, cocon,nce an t.hna| The production of hybrid
crossyaneties are common n ®aize m d £°, Rt & inb reds by close
varieties involves the following s ep _• sent j n na turai populations in
inbreeding, selflng or from ‘ h *P?°»4 p j 0 f jnbreds, first by phenotypic
low frequencies (less than 10 ) > e „ anc j s | ng j e cross test for SCA
evaluation, follower by topcross test _ production of hybrid
and for prediction of d ° ub /f c?st“or in U S.A., cytoplasmic male
seed , using genetic male ^ eT t e rfiity e z maize, jowar. bajra, sunflower,
e—lion and/or
pollination, e.g ., cotton in India, tomatoes m Europe.
Inbreds may be tested for GCA i^ahoXdle!
is known aseaWyrcrrwg^ Seletof ^ ' Hon bafkcross method, convergent
Inbreds may be improved b} peaig - . me thod and pedigree selection
LTebetr^l/u^r^lte^ecr.op. is a promising scheme and should find
a greater uleinthe breeding of hybrid var.eties m future.
esthetic varieties are produced by crossing in all possible combinations
u y „r L- /hat combine well with each other and are maintained by
SSriBSr i : Safe
resulS crop or bymaktng all possible single crosses and mixing their seed ;
and multiplication of the seed thus produced.
Synthetic varieties offer many advantages over hybrid varieties : they are
practically feasible means of exploiting heterosis in species where poihnatmn
control is difficult ; seed production is simpler and cheaper , farmer can sa\e
h?«o2n^ -thev serve as germ plasm reservoirs and they maybe expected
to perform better ihan hybrids in a variable environment However synthetics
exploit-only the GCA portion of heterosis and cannot utilize that part ot
heterosis produced due to SCA,
The performance of synthetics may be improved in the following ways :
bv increasing the number of parental lines, but this cannot be done beyond #
cenain potaf wihtout adversely affecting the nerfcrmanc* ; by
r^rfnrmance of Svn, generation, which depen is on th« GC A of tne. parental
fines [and by inereasfng the performance of the parental lines which may be
easily done by using short-term inbreds or noninbreds as parents.
In species where pollination control is difficult, combining ability may be
estimated by open-pollinated progeny test, topeross test, polycross test or sing e
cross test. Polycross test is the most commonly used. The seed of synthetic
varieties is maintained by open-pollination in isolation, but they may be
reconstituted from parental lines at regular intervals.
QUESTIONS
1. DeSne the following : hybrid varieties, genetic emasculation, single cross,
testcross, topeross, double cross, tester, detasselling, population cross,
synthetic varieties, composite varieties, germplasm complexes.
2. Give a brief historical account of the development of hybrid varieties witk
special reference to India.
3. Describe briefly the various operations for the production of hybrid
Hybrid and Synthetic Varieties
355
•V "f*
4. Describe the various methods of hybrid seed production.
7 - D ”” b * “»
S. Discuss the achievements through hybrid varieties io cross flnf i „if^ir
nated species with a special reference to the indfan ex^rience P0, ‘"
9 ' '^L Sh )VT, O or he f ?r 1 '° Win8 : (i lP. rediction of double cross perfor-
™“. nc ®’ V't «;><:. of self-incompatibility in hybrid seed Droduetinn
Ui.) hybrid vaneues, (iv) synthetic varieties, (v) order of inbreds in 1
d ruble crO'S, (vi) second cycle inbfeds, and (vii) test for GfA in crw.,.;*,
wh^re pollination control is difficult. m # species
;0 - Bn-fly describe the various operations in production of synthetic varieties
maintaiaed ? Brk0y describ * «“ achievements
”■ s?
the performance of synthetic varieties. gS improvement of
11 ”S!Jl£Sj„!S'vSS£. ,nd < “™ rl “ »f «i«ic
Suggested Farther Reading
ALLAR New'york 60 ' Princi P Ies of piant Breeding. John Wiley and Sons, Inc.,
Chase,^S.SL 1 96^Monoploid and monoploid derivatives of maize. Bot. Rev.
Gama, EE G. and Hallauer, A.R. 1977. Relation between inbred and
hybrid traits in maize. Crop. Sci. 17 : 703-706.
HAYES ^S K A. 1963 - A Pr ° feSSOr ’ s Story of Hybrid Corn - Burges, Minneapolis,
Simmonds^N.W^ 1979. Principles of Crop Improvement. Longman, London,
it'll
w
CHAPTER 20
Clonal Selection And Hybridization
Some agricultural crops and a large number of horticultural
are asexually propagated. Some common asexually progagated
&fe sugarcane (S. officinarum), potato (S. tuberosum )» sweet
potato (/. batatas), Colocasia (Taro), Arum , , Dioscorea (yams),
MtfttHO; ginger ( Zingiber sp.}> turmeric (C. domestica ), banana
[Mus'd paradislaca) etc., almost all the fruit trees, e.g mango
[Mangier a indica ), citrus (OVray 57?.), apples (P. mo/wj), pears
(F. communis ), peaches (P.persica), litchi (LftcA/ chwensts), loquat
(Eriobotrya japonied) etc., and many ornamentals and grasses. Many
of these crops show reduced flowering and seed set, e.g., sugarcane,
potato, sweet potato, banana etc., and some varieties of these crops
do not flower at all* But many of these crops flower regularly and
show satisfactory seed set. However, they are propagated asex ually
to avoid the effects of segregation and recombination, both being
the inevitable consequences of sexual reproduction. Segregation - and
recombination produce new gene combinations due to which tne
progeny differ from their parents in genotype and phenotype. Asexual
reproduction, on the other hand, produces progeny exactly identical
to their parents in genotype because the progeny are derived from
vegetative cells through mitosis. The advantage of asexual reproduc-
tion is immediately clear : it preserves the genotype of an individual
indefinitely. It must be noted that this does not depend on the homozy-
gosity of the genotype of an individual. Any genotype is preserved and
maintained through asexual reproduction. In contrast, self-pollination
preserves and maintains only homozygous genotypes giving rise to
purelines.
Characteristics of Asexually Propagated Crops
Asexually propagated crops generally have the following
characteristics.
1 . A great majority of them are perennials, e.g., sugarcane, fruit
trees etc. The annual crops are mostly tuber crops, e.g., potato,
cassava {M. utilissima), sweet potato etc.
I
CnlottQl Selection ffykridizpibn 357
2. Many of them show reduced flowering and seed set. Many
varieties do not .flower at all. Only the crops grown -for fruit,
particularly where good fruit set depends upon seed formation,
show regular flowering and satisfactory seed set.
3. They are .invariably cross-pollinated.
4. These crops are highly heterozygous and show severe inbreed-
ing depression.
5. A vast majority of asexualiy propagated crops are either poly-
ploids, e.g., sugarcane, potato, sweet potato etc., or have poly-
ploid species. or varieties.
■6. Many species are interspecific hybrids, e.g., banana (M. ppra-
disiacaX sugarcane, Rqbus etc.
7. These crops consist of a large number of clones , that is, pro-
geny derived from a single plant through asexual reproduction.
Thus each variety of an asexualiy propagated crop is a cjope.
Clone
!
I
{
1
A clone is a grciifp of plants produced from a single plant through
asexual reproduction. Thus asexualiy propagated crops consist of
a large number of elopes. Therefore, these crops are also Jcnown as
clonal crops. All the members of a clone have the same, genotype
as the parent plant. As a result, they are identical with each other
in genotype. Consequently, the phenotypic differences within a
clone do not have a genetic basis and are purely due to the environ-
mental effects. A selection within a clone is tbps Useless. The various
characteristics of a clone are summarised below.
1. All the individuals belonging to a single clone are identical in
genotype. This is so because a clone is obtained through
asexual reproduction which involves mitotic cell division only.
Genetic variation in the progeny of a plant is produced chiefly
by segregation and recombination which occur during meiosis
only. Thus the genotype of a clone is maintained indefinitely
without any change.
2. The phenotypic variation within a clone is due to the environment
only. This is so because all the individuals belonging to a
single clone have the same genotype.
3. The phenotype of a clone is due to the effects qf genotype (G),
the environment IE) and the genotype x environment interaction
(GxE) over the population mean (/*-). The phenotype (P)
of a clone may be expressed as P—p+G +E-\--GE. Thys the
phenotypic differences among clones would be partly due to
their genotype and partly due to E and GiTcorppooents. flenee
the efficiency pf selection among clones, as among putplines,
would depend upon the precision with which the E and
components of phenotype are estimated (Chapter 4),
4, Theoretically, clones are immortal, i.e., a done can be main-
tained Indefinitely through asexual reproduction. But clone*
'a
358
Plant Breeding : Principles and Methods
usually degenerate due to viral or bacterial infections. A clone
may become extinct due to its susceptibility to diseases oi
insect pests. Further, genetic variation may arise within a
clone changing its characteristics.
5. Generally, clones are highly heterozygous and show severe loss
in vigour due to inbreeding.
Genetic Variation within a Clone
Genetic variation within a clone may arise due to somatic
mutation, mechanical mixture and occasional sexual reproduction.
Mutation. Somatic mutations are also known as bud mutations. The
frequency of mutations is generally very low (10~ 6 — 10~ 7 ).
Ordinarily, only dominant mutations would be expressed in the soma-
tic tissues because recessive mutations can be expressed only in the
homozygous condition. A mutant allele would be homozygous only
when (I) both the alleles in a cell mutate at the same time producing
the same mutant allele, or (2) the mutant allele is already in the
heterozygous condition in the original clone. Both these events are
possible, but they are expected to be rare. Therefore, we may
conclude that dominant bud mutations express themselves more
frequently than recessive ones. Another aspect of bud . mutations is
that they often produce chimeras, Le„ individuals containing cells
of two or more genotypes. However, it is not a great problem
because* normal plants, i-e., nonchimeras, may be produced from
chimeras through one of several techniques.
Bud mutations make possible the selection of buds to establish
new desirable clones, the process being known as bud selection. Bud
selection is of some importance in the improvement of perennial
crops like fruit trees, or of those crops where flowering does not
take place. It requires a large number of piapts to be observed and
several trained persons to detect the mutant buds. Thus bud
selections are generally practised in commercial plantations and net
in the breeder’s nurseries. Further, the discovery of desirable bud
mutations has been more of an accident than a result of planned
search for them.
Mechanical Mixture. Mechanical mixture produces genetic varia-
tion within a clone much in the same manner as in the case of
purelines.
Sexual Reproduction. Occasional sexual reproduction would lead to
segregation and recombination. The seedlings obtained from sexual
reproduction would, therefore, be genotypically different from the
asexual progeny.
It is evident that old clones would tend to become variable, at
least in the annuals and biennials, e.g. s potato. A selection
within an old clone may be useful in establishing a new and more
desirable clone. For example, Kufri Red potato is a selection from
Darjeeling Red Round ; if. was derived from a single disease-free
plant. Similarly, Kufri Safed potato is a selection from Phulwa.
Particulars Clone
Mode of pollination in Cross-poSHna*
the crop species where tion
they occur
Natural mode of repro- Asexual (in
duction in such species most cases)
Genetic make-up of the Heterozygous
plants in natural popu-
lations of such species
Obtained through Asexual repro-
' auction from a
single plant
• Pureline Inbred
Cross-poili- Cross-pollinatior*
nation
Sexual
Homozygous Heterozygous
Natural self* Artificial self* '
pollination pollination (or
from, a single other forms, of
homozygous inbreeding) and
plant selection for
several genera-
tions
Natural self- Artificial self-
pollination pollination or
close inbreeding
Identical Almost identical
Asexual
reproduction
Identical
Yes ' No (Used in deve-
loping hybrid or
synthetic varieties)
Homozygous ■ Almost ’ ■ homozy-
gous
Plants • Plants, animals':
directly as
The genetic make-up of
plants within a variety
Organism where found
Heterozygous
Plants
3€0 Plant Breeding : Principles and Methods
in patato occur at a frequency of !0~ 3 in some clones, and are a
serious problem in the maintenance of these' clones.
Viral' Diseases. Viral disease a^e easily .transmitted through vegeta-
tative propagules. 'Therefore, they easily spread with time in the
case of clonal crops. Viruses are perhaps responsible for more cases
of clonal degeneration than any other single cause. Many clones in
..potato have been lost due to virus infections.
Bacterial Diseases. Some cases of clonal degeneration are the results
of bacterial infections. The ratoon stunting disease of sugarcane
(S. qffiqinpmm) is caused by a small bacterium. There are other
similar instances where a bacterium is responsible for the degenera-
tion of clones.
Another reason for the deterioration of clones may be somatic
mutations but they are not likely to be the common cause. The
vegetative reproduction itself does no! affect the vigour or the pro-
ductivity of clones. On the other hand, asexual reproduction keeps
intact the specific gene combinations which make a clone superior
to or more desirable than other clones. It is likely that with further
investigations more and more cases of clonal degeneration would be
found to be the results of bacterial and viral infections.
Methods of Improvement of Asexuaily Propagated Crops.
Asexually propagated crops differ from the sexually propagated
ones in one basic aspect that the breeding materials and the commer-
cial varieties are maintained by asexual reproduction in the case of
%$& former. ' This provides some unique advantages and opportuni-
ties in their breeding, A single outstanding plant selected from a
population forms the basis of a new variety. The breeding behaviour
or genotype of the plant is not important since there would be no
further sexual reproduction . The outstanding plant may be selected
from an old unimproved variety, an improved variety that has
become variable, or from a population produced by crossing two
or more clones. Thus the breeding of clonal crops has two .well
defined phases which are the same as those in the case of sexually
reproducing crops. These phases are : first > the utilization of already
existing variability, and the creation of variability through hybridiza-
tion where it does not exist, and second , selection of the best genotvpe
to produce a superior clone or variety. The procedure of selection
is known as clonal selection ■ since the selected plants are used to
produce new clones.
We shall first gamine f he main features of clonal selection
. end later discuss the procedure for creation of variability, through
fcybridkatioxi.
CIoimI Selection
< The phenotypic value of a plant or a clone is due to the effects
of its genotype (G), the environment (IF) and the genotype x environ-
ment interaction (GE). Of these* qply.ttjeC? effects are heritable
„«nC ? t r X re, a nH nth and interaction effects are
noohentable and cannot be selected for. Therefore a fnr
quantitative characters based on the observations on Jingle riaSs
is high y unrehabte. In fact, plants selected in this way may be no
Si. tan hT ( random sample. Further, p. selection for characters like
yielding ability etc. on the basis of unreplicated clonal plots would
often be misleading and unreliable. The value of a clone can be
reliably estimated only through replicated yield trials. However
selection for highly heritable characteristics, such as plant height’
days to flowering, colour, disease resistance etc ’are easv and
Few to several hundred
sekefed
superior plants
MIXTURE QF CLONES
SECOND
YEAH
(i) Clones from the selected plants
grown separately
(ii) Desirable clones selected
THIRD
YEAR
(i) Prelin inary yield trial with standard
checks
(n) Selection for quality* disease resis*
trance etc. Disease nurseries may be
planted.
(HO Few outstanding clones selected
PRELIMINARY yield trial
(i) Multilocation yield trials with stan-
dard checks
(it) Best clone Identified for release as a
new variety
replicated yield trials
(I) The best clone released as a new
variety
NINTH
YEAR
(ii) Seed multiplied for distribution;
SEED MULTIPLICATION
A generalised scheme for clonal selection
species. ■ >
In asexually propagated
362
rtam Breeding : Frinciples and Methods
In view of these considerations, in the earlier stages of clonal
selection, when selection is based on single plants .or single plots,
the emphasis is on the elimination of weak and undesirable plants or
clones. The breeder cannot reasonably hope to identify superior
genotypes at this 'stage. In the later stages when replicated trials are
the basis of selection, the emphasis is to identify and select the
superior clones. The various steps involved in clonal selection are
briefly described below and are depicted in Figure 20 J.
First Year. From, .a mixed variable population, few hundred to few
thousand desirable plants are selected. A rigid selection can be done
for simply inherited characters with. high heritability. Plants with
obvious weaknesses are eliminated.
Second Year. Clones from the selected plants are grown separately,
generally without replication. This is because of the limited supply
of propagating material for each . clone, and because of the large
number of clones involved.
The characteristics of clones will be more clear now than in. the
previous generation when the observations were based on individual
plants. The number of clones is drastically reduced, and Inferior
clones are eliminated. The selection is based on visual observations
.and on the breeder’s judgement of the value of clones. Fifty to one
hundred clones are selected, on the basis of clonal characteristics.
Third Year. Replicated preliminary yield trial is conducted,. A
suitable check is included for comparison. Few superior perform-
ing clones with desirable characteristics are selected for multilocation
trials.
At this stage, . selection for quality is done. ' If necessary, sepa-
rate disease nurseries may be planted to evaluate disease resistance
of the selected clones.
Fourth to Eighth Years. Replicated yield trials are conducted at
several locations alongwith a suitable check." The yielding ability,
quality and disease resistance etc. of the clones are rigidly evaluated.
The best clones that are superior ; to the check in one or mo re
characteristics are identified for release as varieties.
Ninth Year. The superior clones .are multiplied and released as
varieties.
Merits of Clonal Selection
L It is the only method of selection applicable to -clonal crops.
It avoids inbreeding depression, and preserves the gene combs*
nations present in the clones.
2. Clonal selection, without any substantial modification, can be
combined with hybridization to generate the necessary varia-
bility for selection.
3. The selection scheme is useful in maintaining the purity ol
clones.
Clonal Selection And Hybridization
363
Demerits of Clonal Selection
L This selection method utilizes the natural variability already
present in the population ; it is not devised to generate
variability.
2. Sexual reproduction is necessary for the creation of variability
through hybridization.
Hybridization
-V
>
Clonal crops are generally improved by
desirable clones, followed by selection in the FT progeny and in the
subsequent clonal generations. Once the F 3 has been produced, the
breeding procedure is essentially the same as clonal selection. The
improvement through hybridization involves three steps : (1) selec-
tion of parents, (2) production of Fi progeny, and (3) selection of
superior clones.
Selection of Parents. Selection of parents to be used in hybridi-
zation is very important since the value of F t progeny would depend
upon the parents used. Parents are generally selected on the basis
of their known performance both as varieties and as parents in
hybridization programmes. The performance of a strain in hybrid!*
zation programmes depends on its prepotency and general combining
ability. • It would be highly, desirable to know the relative values^
of GGA and SGA in the crop to be improved. If GCA is more
important, a small number of parents* with good GCA should be
used in hybridization programmes. On the other hand, when SCA
is more important, a large number of parents should be used to
produce a large number of Fi families.
A recent , suggestion is to .partially inbreed the parents to be
used in hybridization programmes. Clonal crops show 1 severe
inbreeding depression, but it is expected that one 'generation of
selling or 2-3 generations of sib mating may not reduce vigour and
fertility too severely. Inbreeding may enable the breeder to identify
plants that would have a - greater concentration of desirable genes.
These plants may be more prepotent as parents than the highly
heterozygous clones. This practice is gaining some favour with
plant breeders. /AbAy .y.Cv'A:
Production of F x Progeny. Generally, clonal crops are cross-pollina-
- ted and they may show self-incompatibility. The selected parents
may be used to produce single crosses involving two parents or an
equivalent of a polycross involving more than two parents.
Selection among P* Families. When the breeding value of the parents
is not known, and the relative contribution of GCA and SCA is not
available,* a large number of crosses have to be made in order to’
ensure that atle ast some of the crosses would produce outstanding
progeny in F 5 , This is particularly true in a species where crop
improvement has not been done or has been done at a small scale. In
such cases, it would be cumbersome to evaluate a large number of F*
progeny in detail* To avoid this, generally small samples of several.
Plant Breeding : Principle s and Methods
Selected clones are bybri-
Sized
FIRST .P^ENT^i
YEAR ' CLONES
CLONE B
CLONE A
(i) Several thousand (5,000-
20,000) seedlings derived
through sexual reproduction
space-planted
(ji) 500-2,000 superior plants
select'd
oooooo oog ooo
OOOOOO GOO ooo
ooo OOO oooooo
OOOOOO OOOOOOl
OOOOOOOO OOOO
{SEXUAL
PROGENY)
(i) Individual clones from
selected Fi plants are grown
with suitable checks'
(II) 50*200 superior clones
selected
YEA#
U) Preliminary yield- trial
with appropriate, checks
(ji) Few outstanding clones
selected for multi location
testing'
FOURTH
YEAR
CLONES
(i) Multiiocation yield trials
with standard varieties as
checks
(ii) . Clones superior to the
checks arc identified for
release as new varieties
'FIFTH-
NINTH
YEARS
CLONES
_ Z — (i) Outsanding c!one(s) re-
clocks ~ - leased as a new variety
, — ; (h) Seed multiplication for
'Z distribution among farmers
A generalised scheme for breeding of annual asexually propagated
species through hybridization
fmm
v%m •
Colonial Celeb (idri Arid HyBrtdizhTfon
3 6 $
Fi populations are gidwii. the general worth of individual F,
families or populations is estimated visually. The presence of out!
standing individuals in these families is also noted. Inferior families
are eliminated. Promising families with outstanding individuals are
then grown at a much larger scale for selection, the procedures
designed to save time, space and labour by planting only small
populations of a large number of crosses at the preliminary stage.
Selection within Ei Families. The selection procedure within F.
families is essentially the same as that in the case of clonal selection
The various steps involved in the breeding of clonal crops through
hybridization are listed below (Fig. 20.2). From step 2 onward
these should be read alongwith the steps in clonal selection.
First Year. Clones to be used as parents are grown and crosses are
y Made to produce Fj progeny.
Second Year. Sexual progeny from the cross, /.<?., seedlings obtained
from seeds, are grown. Undesirable plants are eliminated. Few
hundred to few thousand desirable piants are selected.
Third Year. Clones from individual plants are grown separately
Poor and inferior clones are eliminated. Upto 200 superior clones
may be selected for preliminary yield trial.
Fourth Year. A replicated preliminary yield trial is conducted. A
suitable check is included for comparison. Few outstanding clones
are selected for trials at several locations.
Fifth to Ninth Year. Replicated yield trials are conducted at several
locations. A suitable check is included for comparison. One
■ ! or a few outstanding clones are identified and released as new
Jg varieties.
Tenth Year. The clones released as varieties are multiplied and
distributed among farmers.
Interspecific Hybridization in the Improvement of Clonal Crops
Interspecific hybridization has been successfully used in the
improvement of clonal crops. Many varieties of Rub us, Malus, straw-
berries etc. are interspecific hybrids. Potato and sugarcane, two of
the clonal crops that concern us the most, have also benefited from
interspecific hybridization. Potato variety Kufri Kuber was develop-
ed from a complex cross ( Solanum curtilobum XSolanum tuberosum)
xSolanum andigena. This variety shows much less clonal degenera-
tion in the plains than the variety Up to-date. Generally,
interspecific crosses are made to transfer specific characters, such as,
disease resistance, from the wild species to the cultivated potato,
y S. tuberosum. For example, Solanum demissum has been extensively
| used as a source of late blight resistance.
j Sugarcane is a special case. All the sugarcane varieties ad'# la'
cultivation have been developed from complex crosses between
Sdcehdruni officlmrttni, S. barberi, S. fobustum and S. spontaneunf.
| S. spontaneum has- beet* used to combine its hardiness arid disease
366 Plant Breeding : Principles and Methods
resistance with hieh' sugar content and high yielding ability of 5.
offkinamm. These" noblised canes give higher yields of both cane and
sugar and have higher sugar content than the noble canes themselves
Similarly, interspecific hybridization has been a useful tool in the
improvement of other clonal crops. The principal reason tor such
a creat success of interspecific hybridization m the clonal crops is
their asexual reproduction ; this completely avoids segregation and
recombination. Another reason is that most of them are not s-ed
crops, hence flowering and fertility are not essential for tneir success
as varieties.
Problems in the Breeding of Asexualiy Propagated Crops
It mav appear that the breeding of asexualiy propagated crops
is simple ‘but in actual fact it is not. There are several problems
necoiiar to clonal crops which are difficult to resolve. There are
three major problems in their breeding: (!) reduced flowering
and fertility, (2) difficulties -in genetic analysis, and (a) perennial
life cycle.
Reduced Flowering and Fertility. Clonal crops grown for vegetative
nar»s generally show reduced flowering ; some varieties go not
flower” at ail', e g . sugarcane, potato,, sweet potato, Colocasia etc.
These crops also show considerably sterility due to polyploioy
(potato, sugarcane, sweet potato etc.), complex interspecific n ^? n "
ditv and cvtoplasmic male sterility. Tne reduced flowering arm hig.«
sterility seem to be a result of correlated responses to selection for
increased yields of vegetative parts.
In fruit crops, flowering does occur, but seed set is generally
much reduced. The possible causes for reduced seed set are poly-
ploidy (pears, Rubus, some apples, bananas, strawberries etc.) and
interspecific hybridity ( Rubus, strawberries, bananas etc.). In addition,
oarthenocarpv occurs in many fruit trees, e g., pineapple. Citrus, _ tig
etc ’ Facultative apomixis occurs in mango and Citrus, which is a
nuisance when the breeder desires to obtain sexual progeny from
crosses for further selection. Because of reduced flowering and seed
set, many desired clones cannot be used in hybridization programmes
of’ many clonal crops.
Difficulties in Genetic Analyses As discussed earlier, estimates of
GCA and SCA are very helpful In the selection of suitable parents
for hybridization programmes. Genetic analysis in the most of
clonal crops is very difficult due to one or more of the following :
(1) reduced flowering, (2) sterility and (3) perennial life cycle.
Perennial Life Cycle. Perennial life cycle drastically increases the
time required for obtaining sexual progeny for the genetic analysis
of clones. Genetic analysis in most of the fruit trees is generally not
attempted for this reason. Replicated yield trials, so familiar in the
case of annuals, are not possible in the case of perennials.
Fig. 20.3. Tubers of ‘Kufri Chandramukhi', an early maturing
variety of potato developed through hybridization.
frost resistant. In case of sugarcane, ail the varieties have been deve-
loped through interspecific hybridization. Some prominent varieties
- of sugarcane are CO 1148, CO 1158, COS 510, CO 975, COS 109,
- €0 541 etc.
if* , SUMMARY •
Many agricultural and a large, number of horticultural crops are
J asexually propagated. The varieties in such crops are clones i.e., asexual
s progeny of single plants. _ Clonal crops are highly heterozygous and show
severe inbreeding depression. They are cross- pollinated, perennial, usually
S polyploid, many are interspecific hybrids and show reduced flowering and
I fertility. , ^
Clonal Selection And Hybridization
Achievements
Clonal Selection. Several varieties of clonal crops have been develop-
ed either by bud selection or by clonal selection from heterogeneous
clones. Kufri Red potato is a clonal selection from Darjeeling Red
*°““ d c : " 4 w . as developed from a single disease-free plant. Similarly,
kufn Safed is a clonal selection from the potato variety Phulwa
Bud selection has 1 been done in several fruit trees. For example*
Bombay Green banana is considered £o be a bud selection from
Dwarf Cavendish ; Pidi Monthan from xVfonthan, and High Gate
from Gross Michel. *
Hybridization. A large number of varieties of clonal crops have been
developed through hybridization. Some prominent potato varieties
developed through Hybridization are Kufri Alankar, Kufri Kuber
Kufri Sindhun Kufri Kundan, Kufri Chamatkar, Kufri Chandra-
mukhi etc. Kufri Jyoti is late blight resistant and Kufri Sheetman is
368
Plant Eroding : Principles and Methods
Clones arc homogeneous, stable and theoretically Immortal. Genetic
variation within a clone may arise by mechanical mixture, mutation or sexual
reproduction. Clones may degenerate due to infection by bacteria' and viruses,
and possibly due to mutation. Clonal crops are improved by clonal selection
within 'a done showing variation or by hybridization followed by clone!
selection. Interspecific hybridization has been extensively- used for the improve
mmt of several clonal crops. Clonal selection essentially consists of selection
of superior plants to produce new improved clones. Clonal field crops, such
as potato and sugarcane, have been improved by clonal selection as. well as
hybridization. -X
QUESTIONS
1. What is a clone? What are the main characteristics of clones?
Compare cSohes r Jnbreds and pipelines.
2. Discuss the various sources of genetic variation within clones. ffo#~
would you create variation in a clonal crop ?
3. What is clonal selection ? Describe the method for clonal sgfofticttL
Discuss its merits, demerits and achievements.
4. What are the various steps involved m the improvement ofclona!
crops through hybridization ? Describe these steps in brief. Discuss
■the problems and achievements' of hybridization in the improvement of
the clonal crops.
5. With the help of suitable examples, describe the contribution of
interspecific hybridization in the improvement of asexual ly reproduc-
ing species.
6. Discuss in^ detail the salient features of improvement programmes in
the following clonal crops : (i) potato, (ii) sugarcane.
7. Explain clonal degeneration. Does clonal degeneration have a genetic
basis ? Discuss the above in the light of the factors responsible for
clonal degeneration.
Suggested Further Keadteg
FErwerda, F.P. and Wit, F. (eds.) 1969. Outlines of Perennial Crop Breeding
in the Tropics, Veeman and Zonen, Wageningen, The Netherlands.
?oelhman, 3.M. and Borthakur, D.N. 1969. Breeding Asian Field Cr op« with
Special Reference to Crops of India. Oxford and IBH Publishing Co.
New Delhi.
Simmonds, N.W. 1961. Mating systems and breeding of perennial crops
Advancement Sci. y London, 18 : 183-186.
Simmonds N.W 1979. Principles of Crop Improvement. Longman, London
and New York.
CHAPTER 21
Breeding for Disease Resistance
V Disease is an abnormal condition in the plant produced by an
organism. The plant affected by a disease is known as host, while
the organism that produces the disease is termed as pathogen.
Clearly, abnormalities produced by nonbiological environment or
by the genetic factors present in the host are not diseases. Diseases
are produced by a variety of organisms from both plant and animal
kingdoms, viz., fungi, bacteria, viruses, nematodes and insects.
Different crops are attacked to different - degrees by the different
Jcktds of pathogens, but it may be emphasised that all the crop
species are attacked by them. For example, cereals suffer from
epidemics of air-borne fungi, most solanaceous crops are infected
severely by viruses, cotton is damaged by many insects and so on.
In the order of their importance (based on the damage caused by
j them), the pathogens may be listed as fungi>bacteria>viruses>
> nematodes =insects. Much of the breeding effort has been directed
f against diseases caused by fungi, which may be greater than the
effort against all the other pathogens put together. Therefore, our
discussion would be primarily based on the knowledge of fungal
diseases. Breeding for insect resistance is dealt with in some detail
in the next chapter (Chapter 22).
LOSSES DUE TO DISEASES j
Diseases reduce total biomass (dry matter) production by I
the crop in one or more of the following ways : killing of plants, j
killing of branches, general stunting, damage to the leaf tissues and |
damage to the reproductive organs including fruits and seeds. The I
degree of damage would 'depend upon the intensity of infection but
1 both need not be proportional. Satisfactory scales are available for
-S* monitoring the intensity of infection, but there is no scale available
f or assessing the economic loss caused by a disease. However, |
there is no doubt that diseases reduce biomass and consequently ;
! yield Often the loss due to diseases may range from a few to 20 j
} o r 30 per cent ; in cases of severe infection, the total crop may be
! j ost ^ striking effect of diseases is the disappearance from culti-
j vation of otherwise excellent but susceptible varieties. Wheat
Plant Breeding : Principles and Methods
(T. aestivum) variety Kalyan Soaa dominated the Indian agriculture
for about a decade, but bad to be abandoned as it became suscep-
tible to leaf rust. Similarly, another wheat variety Janak was
eliminated from cultivation due to its susceptibility to Karnal bunt.
Another feature of diseases is that for each crop species, the impor-
tance of different diseases keeps on changing. Diseases that were
minor in the past become important due to the changes in crop
varieties and agricultural practices, e.g., Helminthosporium leaf blight
in maize (USA), and leaf hoppers and tungro virus in rice (India).
New diseases may also be introduced alongwith plant materials
introduced from other countries quarantine measures are directed
against, such movements of diseases and insect pests (Chapter 2).
HISTORY OF BREEDING FOR DISEASE RESISTANCE
Theophrastus, in the third century B.C., noted that cultivated
varieties differed in their ability to avoid diseases. That diseases
are produced by a . pathogen was conclusively shown by Benedict
Prevost ; he showed that wheat bunt was produced by 'a fungus.
During the middle of nineteenth century, various workers eg
Andrew Knight, noted that crop varieties differed for disease’ resis-
tance. In 19 05, Biffen demonstrated that resistance to yellow rust
■in wheat was governed, by a recessive gene segregating in the ratio
3.: 1 m F a . Subsequently, in 1904, Blakesfee described mating-tvne
differentiation in Rhizopus.
In 1894, Erikson showed that pathogens, although morphologi-
cally similar, differed from, each other in their- ability to attack
different related host species. Later, in 1911, Bairns showed that
different isolates of a microorganism differed in their ability to
attack different varieties of the same host species ; this finding:.- is the
basis for physiological rgces and/or pathotypes. It was subsequently
established that the ability of a pathogen to infect a host strain i e
pathogenicity, is genetically determined. Thus both the ability of the
host to resist invasion by a pathogen as well as the ability of a
pathogen to invade its host are genetically controlled. In 1955
Flor postulated the hypothesis of gene-for-gene relationship between
host and pathogen, which holds true in most of the cases and is
widely accepted.
. The information available so far clearly shows that the disease
°L h ° St . 1S ^ a , functi °n of the host genotype alone!
Th. w ? et ?T?-» b . y genotype of the pathogen as well!
The host strains differ in resistance, while those of the pathogen
iffer m pathogenicity ; both variations have genetic basis The
u remark - abIe capacity for generating new variation in
ThS ? b eu va J iety . °f. reproduction methods and mutation.
Thus the task of breeders is not only to develop varieties resistant' to
of ,b f I»«"W but S to S
to face . .the challenge to be posed by the new pathotypes in future
Breeding for Disease Resistance
371
MECHANISMS FOR THE GENERATION OF VARIABILITY
IN PATHOGENS
Pathogens have a wide range of flexible mechanisms for
producing variability. The following discussion is based on fungi
but many of the features and the conclusions drawn are applicable to
Fig. 21.1.
The possible pathways for production of new pathotypes in patho-
genic fungi.
other pathogens as well New variability in fungal pathogens may
'be created in the following ways : (1) mutation, (2) sexual
reproduction, (3) heterokaryosis, and (4) parasexual reproduction
(Fig. 21.1).
Mutation. Spontaneous mutations are the ultimate source of all
genetic variation present. Spontaneous mutations occur at a low rate
of 10“ 6 to 10” 6 . But this rate is large enough when it is 'considered
alongwith the astronomical number of asexual and sexual spores
produced by fungi. .
Sexual Reproduction. Sexual reproduction is a common feature of
most of the pathogenic fungi, but in the pathogens belonging to
Fungi Imperfecti sexual reproduction has not been detected. Sexual
reproduction involves the fusion of two haploid cells, which may
•or may not be sexually differentiated, to produce a cell containing
3.72
Plant Breeding : Principles end Methods
two nuclei, a dicaryon. The duration of dicaryon stage varies-
from a very brief period (Phycomycetes and primitive Ascomycetes>
to the entire life cycle, except for brief diploid and haploid genera-
tions fcmuts). The two nuclei of a dicaryon ultimately fuse to-
produce a diploid nucleus which undergoes meiosis ■ to produce four
haploid nuclei. The haploid nuclei may or may not divide mitotically
before they produce haploid sexual spores, which on germination
give rise to the haploid phase of life cycle.
The two cells that fuse to produce a dicaryon may belong to
the same hypha ; such a fungus species is known as homothallic. In
heterothallic fungi, the fusing cells must be of two different mating
types, hence from two different hyphae. There are two mating
types 4- and which are generally controlled by a single gene
wkh two alleles, mt* ond rht~. But in some cases, two or more genes
are involved and in certain other cases multiple alleles are known.
Matins type is not associated with sexual differentiation ; the
latter is controlled by certain other genes. The same mating type
often produces both the male and female gametes, but fusion would
occur between the gametes from different mating types only. Thus the
mating type in fungi is not sexual differentiation but it is similar to
self-incompatibility genes of higher plants. Hetero thalhsm, thus
ensures the fusion of genetically dissimilar nuclei to produce the
dicarvon while homothallism permits the fusion of cells from the
same hypha. But in the case of homothallism as well, dissimilar nuclei
mav fuse In general, homothallism appears to be less efficient m
the generation of variability than heterothallism. But homothahsm is
common in fungi and is known in all the groups of fungi. This
obvious success of homothallism is a good evidence for the ability of
the system to generate the necessary variability for adaptation and
survival.
The fusion of two haploid, genetically dissimilar nuclei, follow-
ed by meiosis, produces new gene- combinations through segregation
and recombination. Sexual reproduction is the most common means
of recombination of the existing variability., A less frequent, but
equally potent, means of genetic recombination is parasexual repro-
duction in fungi.
Heterocaryosis. In many fungi, e.g., Fungi Imperfecti, the hyphae
are multinucleate during the active growth. These nuclei may be
genetically identical ( Homocaryosis ) or of dissimilar genotypes
f heterocaryosis ). Heterocaryosis is produced by the fusion of vegeta-
tive hyphae which are genetically dissimilar ; this fusion is not
affected by the mating type. Heterocaryosis occurs in nature, and is
of adaptive value. Heterocaryons are generally more vigorous than
homocaryons. During mitosis, the different nuclei of a 'heterocaryon
may get included in different cells producing homocaryons. Thus hete-
rocaryosis is capable of storing a limited amount of genetic variability
which is- released through ‘segregation’ of whole nuclei during
Breeding for Disease Resistance
373
■mitosis ; this process appears to faaveian adaptive significance. There
is evidence that heterocaryosis can change the pathogenicity and
■may even give rise to new races of a pathogen.
Parasexual Reproduction. In parasexual reproduction , diploid nuclei
are produced in vegetative cells from which haploid nuclei are pro -
duced through mitotic irregularities , but not through meiosis . Diploid
cells occur in the heterocaryons at a frequency of J(T 7 as a result of
the fusion of two nuclei present in a heterocaryon. These diploid
cells give rise to hapdoid cells at a frequency of 1(T 8 . Crossing over
does not take place during haploidization of diploid nuclei ; hence a
chromosome is inherited as a single unit. However, somatic recom-
bination does take place in a low frequency, i.e., one chiasma pier
10 nuclei. But somatic recombination is not associated with soma tc
reduction (production of haploid nuclei through mitosis) ; the two
events are independent of each other and may take place in the
same or in different nuclei.
Thus heterocaryosis may produce a different race of the
pathogen through an interaction among the genes two nuclei. But
when it is coupled with parasexual reproduction, it produces new
pathogenic races through recombination of entire chromosomes ;
occasionally there may be a recombination of genes as well (when
somatic recombination has occurred before somatic reduction).
PHYSIOLOGICAL RACES AND PATHOTYPES
The concept of physiological races was introduced by Barrus
in 191 1 . Physiological races are strains of a single pathogen species
differing in their ability to attack different varieties of the same host
species. The varieties of a host species used to identify physiological
races of a pathogen are known as differential host or host testers .
Differential hosts are chosen on the basis of differences in their
resistance to the pathogen, but the genes for resistance present in
them are usually not known. Ideally, each of the differential hosts
should possess a single resistance gene different from those present
in the others. If this were so, n differential hosts would be able to
identify 2” physiological races if there were only two types of host-
pathogen reaction, e,g., resistant and susceptible. If, however, more
types of reaction were identifiable, as is the case in stem rust of
wheat, a still larger number of races can be identified ; the number
of races identifiable would be r n , where, r=number of reaction types
identifiable, and «=number of differential hosts with a single unique
resistance gene. Identification of physiological races can be explained
using bunt (a fungal disease) resistance of Martin and Turkey wheat
varieties (Table 21.1). A set of 18 differential hosts, each carrying a
single gene is available for linseed stem rust, and a set of 12 differen-
tials is available for late blight in potatoes. How many races can
these differentials identify assuming two types of reaction ?
A pathogen race capable of attacking a host strain carrying a
those unable to attack this strain, are known as avirulent. When the'
strains of a pathogen are classified on the basis of their virulence to
known resistance genes present in the host, they are referred to as
pathotypes or biotypes . The ideal .differentials, each carrying a
single distinct resistance gene, would identify pathotypes (Table 21.2).
Table 21.1. The identification of 4 physiological races of Tilletia sp. producing
wheat bunt using two differential wheat varieties.
Differential Host
Reaction to bunt
Race 1
Race It
Race III
Race IV '
Martin
R*
R
S*
s
Turkey
R
S
R
s
*R«resistant, and S= susceptible
Pathotype classification is more precise than that of the physiolo-
gical races. It is because in the case of physiological races the
resistance genes present in the differential hosts are generally not
known, and a differential host may possess two or more resistance
genes. There is a growing trend to change to pathotype classifi-
cation from physiological race classification. It may be pointed out
that pathotype classification is limited by the availability of new
resistance genes and by the evolution of new pathotypes virulent to
the hitherto resistant genes .
It would be .seen that physiological races are in fact pathotypes
if each of the differential hosts carry a single and distinct resistance
gene. But if some of the differentials carry two or more resistance
Table 21.2. Pathotype classification based on a set of 12 differential hosts, each
carrying a single distinct resistance gene (RrRn) to Phytophthora
inf e starts ; the fungus causing potato late blight
Resistance gene Pathotype of potato late blight fungus
present in the -
differential host { P x ) (A) (Pa) (A) (A) (A) (A) (A) (A) (Ao> (AuHAa)
_ ”
A S
A S
A ^
A S
A ' . $
r 7 s
A s
A . S
Rio ■ & .
. . . s
R \ 2 . ^
Note : Only the susceptible reactions are shown. Complex pathotypes carrying
more than one virulence gene, e.g.> P{ u z) or P(i, 5 , 8 * is), are identified
by their virulence against the respective host genes for resistance {e.g. t
Rh A and Ri , A* A* i? J2 , respectively). A susceptible reaction of the
host indicates the virulence of pathotype, while a resistant reaction-
shows avirulence.
Breeding for Disease Resistance
375
,°f T e ,h / n ■«“
Some pathotype differentiation occur^inT M°£ ne fu , gI and viru « e s-
and insects. rs ln s °d- borne fungi, nematodes
GENETICS OF PATHOGENICITY
is synOT»mS'S''Jirai t e n e C e a!> A SLSf.* S’? 0 ?™ “ *«•<* • tost and
is possible onlv fcfta, " the inheritance of virelence
Extensive studies have shown thnl^tb r ® sl f ance £ ene is available,
under genetic control 6 More than^!np^^ VIrU CDCe of a P^ogen is
the control of virulence and at each Ef-K are S^r&lly involved in
In many pathogens el rust and USaally two alic!es -
rather longer life, it has been It fl W - he f e d,car Y ons have a
avirulence* Why the same has nntZ* ^ v,ruIence is recessive to
with a brief dfca,yo,“SI g et Seir“?e V" 1 *?*'?*
such cases, the parasitic ,tL? woo d be S35d Vm£ < ? b?r ,S *! m
of ascospores, virulent and avirnlent i . pi^uces two types
single gene control with two alleles. The inclusio/Sf other varieties
ofSthlgenlcitV^So 6 ^ 0 ^ 536 - 8 mor , e information on the inheritance
Wealthy 'but are av.ntlent toYvards McIntosiT vartety of app]e°'whije
8 rdin ® cr pssed, the resulting ascospores are of 4 tvoes • virulent
Mclnt^ alt } y an 1 MelQtosb ’ viruLt on Wealth virulent on
fovoNerrent aviru ent °. n both (Table 20.3). This clearly shows the
and fecTmosh f Witl?™," 1 th - t COnt [ o1 of Pathogenicity on Wealthy
ana Mcxntosh. With more resistant hosts, a finer analysis of natho-
ty ma r be r done interms of the number of genes governing
pathogenicity. In case of late bliaht potato, 12 different lenes irl
ge^TprSeltTn the ]£?“* ^ tho C0 ™ s P0* d ^ distance
DISEASE DEVELOPMENT
well Zt*rj!° pment 0f dise ? s ? s P roda ced by fungi occurs in four
^. debned ® ta S es : contact, infection, establishment and develop-
S C r!3 aC t- represents the landing of a pathogen on the boS
tissue. Infection is the process by which the pathogen gains entry
Plant Breeding : Principles and Methods
Table 21.3. The inheritance of pathogenicity in the ascomycete Venturia
inacaualts causing apple-scab. The notation of strain A and B is
arbitrary. Note that when only one resistant variety of the host is
used, pathogenicity appears to be governed by a single gene. But
when two different resistant varieties are used, there is evidence
for two genes. More genes may be detected by using more
varieties differing in their resistance genes.
Host variety
Pathogen strain
McIntosh
Wealthy
Remarks
AxB
(ascospores tested on
Wealthy)
R (50% of ascospores)
S (50% of ascospores)
Ratio i : 1
elusion ; or
with 2 alleles
AX B
(ascospores tested on
McIntosh)
(50% of ascospores) R
(50% of ascospores) S
Ratio I : 1, con-
clusion : one gene
with two alleles
AxB
(ascospores tested on
both)
R (25% of ascospores) R
R (25% of ascospores) S
S (25% of ascosppreg) R
S (25% of ascospores) S
Approximate ratio
1:1 : 1 : 1. two
genes each with two
alleles
£*«= susceptible, and /^resistant. Susceptible reaction of the host denotes
virulence of the pathogen, while resistant reaction indicates aviruience.
into the host tissue. Usually, spores germinate and enter the host
tissue through natural openings, such as, stomata, or through
wounds. But many pathogens can enter the host tissue through the
epidermis by producing some hydrolytic enzymes. Both contact and
infection stages are greatly affected by the environment and provide
the means for disease escape. But once the pathogen has entered the
host tissue, the establishment phase begins. In this phase, the
pathogen proliferates and spreads within the host tissue, but the
symptoms of the disease are not visible during this stage. The next
phase, the development phase , is characterised by the development
of characteristic symptoms of the disease and is generally associated
with spore production (or multiplication). The rate of spore
production or multiplication is the crucial factor in the spread of
diseases as the spores serve as inoculum for the uninfected plants.
Usually, disease resistance in a host involves a restriction on the
establishment and, particularly, multiplication. In most cases of
disease resistance, infection and a certain degree of establishment
do take place ,* the resistance usually reduces the degree of establish-
ment and the rate of spore production.
DISEASE ESCAPE
As the name suggests, disease escape refers to the freedom of suscep-
tible host varieties from a disease due purely to the environmental
•breeding for Disease Resistance
377
factors. Disease escape occurs primarily by avoiding contact, but
unfavourable weather conditions may prevent infection. Disease
escape may be a result of the environmental factors, early varieties,
i changed date of planting, change in the site of planting, use of rests-
tant root stocks, balanced application of NPK, control of the disease
carrier and control of the pathogen itself. The disease development
requires a suitable environment for spore germination and infection ;
an unfavourable environment would reduce or prevent the germination
of spores and infection. For example, susceptible varieties of potato
(S. tuberosum) escape late blight attacks in years when the growing
period is relatively dry. Early varieties often escape a disease since
they mature before the disease epidemic occurs, eg., early ground-
nut varieties generally escape ‘tikka* disease and early potato varie-
ties may escape late blight. A changed date of planning may success-
fully avoid a pathogen. Virus-free seed potato is produced by
sowing the crop in October instead of November, the normal planting
time in Jullundur and some other places. The October-planted crop
escapes aphid infestation and, consequently, virus infection since
during October-January the aphid population is very low in these
places. In case of soil-borne fungi, e.g., root rots, wilts etc., and other
-soil-borne diseases, it is highly effective to change the site of planting.
Resistant root stocks to soil-borne diseases have been successfully
used in many fruit trees to grow otherwise susceptible, but
desirable, varieties. A balanced application of NPK is of some
usefulness as potassium is known to enhance the ability of a host to
resist diseases, while N is known to increase succulence and suscep-
tibility. Some diseases, particularly viral diseases, are transmitted
by insects ; the control of the insect vector in such cases is extremely
useful in controlling the disease. The control of yellow mosaic in
mueg (V. radiata) is based on the control of white fly, the vector
for thd yellow mosaic virus. In the end, it may be pointed out that
chemical disease control directed against the pathogen is useful in
protecting susceptible varieties, but it is costly and generally pollutes
the environment.
DISEASE RESISTANCE
Host varieties are classified as susceptible or resistant according
to their response to the pathogen. The various reactions of the hosts
to the various pathogens may be grouped into the following types :
susceptible, immune, resistant and tolerant. In case of some diseases, * j
the host strains either show or do not show the disease, hence ’
the classification of disease reaction is simple. But in most cases, the
host reaction is not so sharply defined and shows a graded variation.
Such reactions or infections are classified according to an arbitrary,
but practically sound, scale in which the disease score may vary from f
0 (with no disease) to 5 or 9 (most susceptible).
Susceptible Reaction. L* the case of susceptible reaction,
development is profuse and is presumably not checked
Plant Breeding- i Principles apd Methods
378
genotype of host. In practice, the susceptible reaction is classifiable
in relative terms only, that is, in relation to the reactionof other
host varieties available and the prevailing environment. Wheat
variety Agra Local is generally taken as the susceptible standard
in scoring wheat rusts.
Immune Reaction. When the host does not show the symptoms
of a disease, it is known as immune reaction. Immunity may result
from the prevention of pathogen to reach the appropriate parts
of the host, e.g., exclusion of the spores of ovary infecting fungi by
closed flowering habit of wheat and barley. But more generally, it
is produced by hypersensitive reaction of the host usually immedi-
ately after the infection has occurred. In hypersensitive reaction, a
group of host cells around point of infection die. This severely
restricts the establishment of pathogen and eliminates, its repro-
duction. Thus in immune reaction, the rate of reproductions of the
pathogen is zero, that is, r=0.
Resistance. Resistance, denotes a less disease development than in the
susceptible variety and is a relative attribute. Infection and estab-
lishment do take place, but growth of the pathogen in the host
tissue is restricted. This results in smaller spots or pustules than in
the susceptible variety. Generally, the rate of reproduction is consi-
derably reduced which limits the spread of disease. In case of
resistance, disease symptoms do develop and the rate of reproduction
is never zero, i e., r> 0, hut it is sufficiently lower than 1 ( the rate- of
reproduction on the susceptible variety ) to be useful. The inhibition of
the growth of pathogens is believed to be nutritional in nature, and
in some cases chemical growth inhibitors may be involved.
Tolerance. Tolerance implies that the host is attacked by the pathogen
in the same manner as the susceptible variety, but there is little or no
loss in biomass production or yield. In certain situations this may be
so, but often this term is used without sufficient evidence. It is
desirable not to use this word unless it has been clearly shown to be
the case. Generally, tolerance is difficult to measure since it is
confounded with partial resistance and disease escape. To estimate
tolerance, the loss in yield or seme other trait of several host varie-
ties having the same amount of disease, e.g., leaf area covered by
the disease etc., is compared. An example of true tolerance is known
in the barely variety Proctor to powdery mildew ; it shows a 13%
yield loss as compared to 20% loss in the varieties Zephyr and
Sultan. Virologists use the term tolerance to express a lack of
symptoms even in the presence of the virus. The lack of symptoms,
however, may be strain-specific, and it does not necessarily mean a
tolerance in terms of damage caused by the disease. Plant breeders
inevitably select for tolerance whenever yield is the basis for selection-
VERTICAL AND HORIZONTAL RESISTANCE
The terms vertical and horizontal resistance were introduced by
Van der Plank and are widely used.
Breeding for Disease Resistance 379
Vertical Resistance. It is also known as race-specific, pathotype-
specific or simply specific resistance. Vertical resistance is generally
determined by major genes and is characterised by pathotype^ specifi-
city, Pathoty pe-specifici ty denotes that the host carrying a gene for
vertical resistance is attacked by only that pathotype which is viru-
lent towards that resistance gene ; to all other pathotypes the host
will be resistant. Thus an avirulent pathotype will produce an
immune response, le., r=Q or close to 0 S but the virulent pathotype
will lead to the susceptible reaction, that is, r=l (Table 21.4).
Clearly , immune or susceptible response in the case of vertical resis-
tance depends on the presence of virulent pathotype. When ike virulent
pathotype becomes frequent, epidemics are common in the case of
vertical resistance.
Table 21.4. The reaction of vertical resistance genes and horizontal resistance
polygenes to three pathotypes of potato late blight. Note that the
major genes produce resistance to the avirulent pathotypes only,
while horizontal resistance produces resistance reaction to all the
pathotypes.
Resistance gene
P(l)
Pathotype
PU)
Remarks
PU)
Pathotype- specific oligogenes
Ri
s*
I*
n
in**
1 0 )
(0) I
R ,
/
s
I ^Vertical Resistance
(0)
w
(.0) |
R*
I *
1
1
(0)
Pathotype-nonspecific polygenes
m
(0 J
R
R
R 1 Horizontal resist-
(<1)
«1)
ed) jance
*5— Susceptible, /—immune, and ^—resistant
** figures within parenthesis represent the reproduction rate of pathogen, that
is, r~0, represents immune response ; r—l, denotes susceptible response ;
while r<l, expressses resistance response.
Horizontal Resistance. It has a number of synonyms, e.g., race-non-
specific, pathotype-nonspecific, and partial or general resistance.
Horizontal resistance is generally controlled by polygenes , that is,
many genes with small effects, and it is pathotype-nonspecific (Table
21.4). That is why it is also known as 'general resistance’. In the
case of horizontal resistance, reproduction rate of the pathogen is
not zero, but it is less than one, i.e., r>0 but <1. Horizontal
resistance, therefore, does not prevent the development of symptoms
of the disease, but it slows down the rate of spread of the disease
in the population.
There is some evidence that polygenes governing horizontal,
resistance operate on, geae-for-gepe basis in a way similar to the
oligogenes of vertical resistance (VR), but often this is not evident
380
Plant Breeding : Principles and Methods
due to the small individual and interaction (resistance X virulence)
effects. More often than not, it is very difficult to separate HR from
VR. Further, HR may not always be more stable than VR. In view
of these, ParlevJiet argues that ‘if so-called HR is difficult to separate
from VR, and if it is not more stable, perhaps the effort to distin-
guish between these two types is meaningless’. Several hypotheses
have been advanced to account for HR ; these are listed below.
1. Ghost Gene Hypothesis . According to this hypothesis, HR
represents the cumulative effect of VR genes which have suc-
cumbed to the corresponding virulence genes of the pathogen,
‘ghost’ or ‘residual’ effect of VR genes. There is some
evidence that such may be the case, but such an effect is always
race-specific.
2. HR may be due to the modifying effects of the genetic back-
ground on VR genes, and due to several VR genes being pre-
sent together. This view is similar to the ghost gene hypo-
thesis.
3. HR is governed by polygenes. Oligogenes act as regulators of
the HR resistance by making the action of polygenes race-
specific.
. 4. The polygenes show gene-for-gene relationship with minor
virulence genes of the pathogen, which means that they are
race-specific. But their effect appears to be race-nonspecific
HR due to the small interaction effects, and due to a relatively
large error variance.
Opinions on the relative values of horizontal and vertical
resistance in crop improvement vary considerably. Their usefulness
is largely dependent on the crop species and the particular disease
in question. In case of potato leaf blight, veritica! resistance is
regarded as a nuisance, and breeders primarily work for improving
horizontal resistance. But in the case of wheat rusts, vertical resis-
tance is the only form of resistance being exploited by breeders. It
may be pointed out that horizontal resistance is becoming increas-
ingly important ( e.g in the case of leaf rust of wheat) and is likely
to play a greater role in crop improvement in the future.
MECHANISM OF DISEASE RESISTANCE
A variety of mechanisms are involved in disease resistance. In
some cases, the basis of resistance is better known than in other
cases. The various mechanisms of disease resistance are :
(1) mechanical, (2) hypersensitivity, (3) antibiosis, and (4) nutri-
tional.
Mechanical. Certain mechanical and/or anatomical features of the
host may prevent infection. For example, closed flowering habit of
wheat and barley prevents infection by the spores of ovary infecting
fungi.
Breeding for Disease Resistance
38!
Hypersensitivity. In a large number of cases, immune reaction is*
j*. due to the hypersensitivity reaction of the host. Immediately after
infection, several host cells surrounding the point of infection die.
This leads to the death of the pathogen or at least prevents its spore
production. The molecular basis of recognition of the pathogen .by
the host is not known. It is believed that phytoalexins are responsible
for hypersensitive reaction. Phytoalexins are specific polyphenolic
or terpenoid chemicals and are produced by the host in response to
the infection by a pathogen or even in response to fungi nonpatho-
genic to the host in question. More than thirty different phytoalexins
have been identified. They are present in many plant species, parti-
cularly in the members of Leguminosae. Phytoalexins are either
fungicidal or fungistatic. This mechanism is found in the case of
biotrophic pathogens.
Antibiosis. It has been suggested that the resistance in several cases,
particularly to insects, is due to the presence of some toxic subs-
tances. But generally critical evidence is lacking and the toxic subs-
tances have not been identified. In cotton, cultivars lacking gossypol,
the polyphenol normally present in the host species, are more sus-
ceptible to boll worm and other insect pests, except the boll weevil
to which they are more resistant than the normal varieties.
Nutritional. The reduction in growth and in spore production is
generally supposed to be due to unfavourable physiological condi-
tions within the host. Most likely, a resistant host does not fulfil the
nutritional requirements of the pathogen and thereby limits its
growth and reproduction. However, a more precise information is
not available on this aspect.
GENETIC BASIS OF DISEASE RESISTANCE
The first study on genetics of disease resistance was that by
Biffen in J905. He reported the inheritance of resistance to leaf rust
of wheat variety Rivet in crosses with some susceptible varieties. In
F 2 , there were 3 susceptible to 1 resistant plants, indicating that
the resistance was controlled by a single recessive gene. Subsequently,
several other studies showed that resistances to various diseases are
monogenically determined, but cases of duplicate, complementary
and other interactions have been reported. Most of the earlier studies
were conducted without taking into consideration the physiological
specialization (pathotype differentiation) of the pathogen, which can
materially influence the conclusions drawn. It is now recognised that
disease resistance may show three different modes of inheritance :
oligogenic, potygenic and cytoplasmic.
Oligogenic Inheritance
In such cases, disease resistance is governed by one or few
major genes, and resistance is generally dominant to the susceptible
reaction. The action of major resistance genes may be altered by
modifying genes in many cases, e.g bunt resistance in wheat. But
in some other cases, modifying genes are not known, e.g., resistance
382
Plant Breeding : Principles and Methods
to Y and X viruses in potato. Oligogenes generally produce immune
reaction. The chief characteristic of the oligogenic disease resistance
is its pathotype - specificity, that is, the resistance gene is effective
against some patho types, while it is ineffective against the others. In
most cases, there are a number of major genes that determine resis-
tance to a particular disease, e.g., more than 20 different resistance
genes are known for leaf rust of wheat, while those for stem rust
resistance exceed 30. The genetics of oligogenic resistance has
advanced by two events occurring repeatedly. These events are :
<1) the discovery of a resistance gene to the prevalent pathotype, and
(2) the evolution of a pathotype virulent to this new resistance gene.
Oligogenic resistance ]is synonymous to vertical resistance.
The inheritance of disease resistance may be explained with the
help of bean (P. vulgaris ) anthracnose resistance. There are two
physiological races, Alpha and Beta, of the pathogen ; Alpha is viru-
lent on the bean variety Robust but not on White Marrow, while Beta
is virulent on White Marrow but not on Robust. When a cross bet-
ween White Marrow and Robust is tested against the Alpha pathotype,
Fi is resistant and in F 2 a 3 : 1 segregation is observed for resistance
and susceptibility. Similar results are obtained when the same cross
is tested against "the Beta pathotype. Thus resistance of each of the
two bean varieties is governed by a single gene which appear to be
different. This can be demonstrated by testing the F 2 from the above
cross against a pathotype (Po) which is a virulent on both the varie-
ties. The F a shows a 15 : I ratio indicating duplicate gene action.
A complementary gene action (F 2 , 9 : 7 ratio) would be obtained if
the F 2 were inoculated with a mixture of the Alpha and Beta races
(Table 21.5). It is evident that the conclusions drawn from inheritance
studies on disease resistance are largely determined by the quality of
the pathogen inoculum ( the pathotype and the purity of the patho-
type)- ■ '
Gene-for-Gene Relationship. The gene-for-gene relationship between
a host and its pathogen was postulated by Flor in 1956 based on his
work on linseed ( Linam usitatissimum ) rust. Subsequent studies have
shown that the gene-for-gene relationship holds true in most of the
cases studied extensively, and is now widely accepted. It has been
found that for every resistance gene present in the host, tlje pathogen
has a gene for virulence. Susceptible reaction would result only when
the pathogen is able to match all the resistance genes present in the
host with appropriate virulence genes. If one or more resistance
genes are not matched by the pathogen with the appropriate virulence
genes, resistance reaction is the result. In most of the pathogens,
virulence is recessive to avirulence.
The gene-for-gene relationship may be explained with the help
of linseed rust resistance. Resistance is governed by a number of
Breeding for Disease Resistance
Table 21.5. The inheritance of reistance to anthracnose in rajma (Phassolus
vulgaris L.)
.■Pathotype
Raima variety
Ratio in F%
White Marrow Robust White Mar- Resistant \ Susceptible
row X Robust
( F ,)
/L46& aaBB AaBb
Alpha (A)** R S R 3 l
Beta (P a ) S R R 3 1
Avirulent (P„) R R R 15 1
Alpha +Beta(P e +/’ i ) S S R 9 7
* Letters within parenthesis denote the genotype of the host. "
* ^GX P(») and P(„) denote the path^type with reference to the resistance
genes A and B . P(.) is virulent toward A, P(,) toward B and PL) toward
neither A nor B.
Note : The conclusions regarding the inheritance of resistence are greatly
affected by the inoculum of the pathogen : incoculum P(„) or P ( 4 )— Fs
ratio 3:1, conclusion— single gene with two alleles ;-inocu3um P(„)— F t
ratio : 15 : 1, conclusion — duplicate gene action ; inoculum P(j + P( 4 )
— Fa ratio 9 : 7, conclusion— complementary gene action.
dominant genes, e.g., N and P. Virulence of the pathogen is' deter-
mined by recessive genes for virulence designated as a. A subscript
denoting the resistance gene toward which a is virulent is also added
The resistance determined by the host gene NN would be overcome
only by the pathotype carrying the virulence gene « NaN • all other
pathotypes will not be able to attack the host. Similarly, resistance
due to gene PP can be overcome by the pathotype with gene apap
only, and that due to genes NN PP by the pathotype c N a N UpUp
only (Table 21.6).
When the host carries more than one gene for resistance, the
pathogen produces a pathotype that has virulence genes for all the
resistance genes. For example, in potato (S. tuberosum ) 12 resistance
genes (/?,, R 2 .R n ) are known for late blight. A pathotype virulent
toward all the 12 genes has been known. But there are some cases,
where the pathogen has not been able to produce a matching viru-
lence gene for the resistance gene. For example, Rx and Ry genes
in potato for resistance to X and Y viruses, respectively, the domi-
nant resistance gene for the Milo disease in Sorghum (jowar) and the
recessive resistance to Victoria blight in oats are not matched by the
pathogen with virulence genes and, therefore, are stable. Some other
examples are wilt resistance in watermelon ( Citrullus vulgaris), resis-
tance to root-knot nematode in Lima beans (P. lunatus ) and resis-
tance to cabbage yellows in cabbage.
An interesting case of a resistance gene unmatched by the
pathogen has been recently reported for leaf rust in wheat. This
Plant Breeding : Principles and Methods
f uncled (L usitatiisimum)
Table 21 .6. ***** 1 ^
384
NN pp
nnPP
NN PP
JsAs
ApAp
As As
apap
asas
ApA p
asas
apap
AsAs
ApAp
As As
apap
asas
ApAp
asas
apap
As As
ApAp
asas
ApAp
As Ax
apap
asas
apap
AsAs
ApAp
As As
apap
as as
ApAp
asas
apap
Disea se reaction
Susceptible
Resistant
Susceptible
Resistant
Susceptible
Resistant
Susceptible
Virulence is due to ‘i* ’ ‘subscript,
tance gene toward which it s virulence toward the resistance gene
^f ’DipW^'genotypes for the parogen are ^'^gj.Q^ygcfus'state, e.g.^
™ ”• — 4 or
ON «P Cp. . y *
resistance allele was to ttealiele for susceptibility. It
variety of wheat, and . n natural races of leaf rust m
has conferred resistance agains U th ainst sever al laboratory
India, Austraha and Canac la ^ bg aQSWered m this
synthesized races. Two vi q identity of this resistance gene .
case. The „ ene f or which the pathogen does not have the
is it an entirely n ? A e or j s jt an extremely strong gene so
will it b= overcomes, the pathogen in dne
Molecular Basis for Gene^orGen^ReladonshjP-^^he^gentaftir-^ne
relationship is due t • P and {hose conditioning patkogene-
g? t QeS Fn°r V examll \ Helminthosporium sacchari, causing leaf blight of
city. For example, « which binds to a protein present m the
sugarcane, produces a toxin whochbmast ^ to the
suscepdiWeTesponse of the host. But in many other cases, such an
MrmSngfar Dittos*; Resistance 385
interaction- produces resistant response. Thus oh the- basis of the
molecular interactions involved ■ Is producing resistant/susceptlble
responses in the host, the geoe-for-geae relationship may be classified
into two general groups : (1) incompatible reaction and (2) compati-
ble reaction.
Table 21.7. The incompatibility (occurring in many biotrophic pathogens)
and compatibility (found in many iieterofcrophic pathogens) types
of genetfor-gene relationship in host-pathogen interactions. One
gene for resistance (dominant) and one gene for virulence
(recessive) are assumed.
Resistance Virulence ge ne
£ ene Incompatibility C ompatihHif ; ~ ~
Pi Pi Pi pi
Pi R* SR R
ri S SR S
* R represents resistant response, while S represents susceptible response of
the host.
Incompatible Reaction. This type of gene-for-geoe relationship is
found in biotrophic pathogens (obligate parasites), e.g., rusts, smuts
etc. The alleles for resistance in the host (R) and those for avirulence
in the pathogen (P) produce specific compounds which recognise
each other ; when these specific compounds interact, they produce
resistant response in the host The alleles for susceptibility (r) in
the host as well as those for virulence (p) in the pathogen either
produce these compounds in a modified state or do not produce
them at all. Consequently, the products of p and r are unable to
recognise, each other, and there is no interaction between them. The
lack of interaction between the products of the genes for resistance*
and pathogenicity produces susceptible reaction in the host. As a
result, three of the for combinations between the two alleles of a
single gene for resistance (host) and the two alleles of a single gene
for" pathogenicity (pathogen) would be susceptible (Table 21.7). Only
one of these four combinations would lead to the resistant response,
since the products of R x and Pi would recognise and interact with
each other. Clearly, this system is akin to the self-incompatibility
system of higher plants.
Compatible Reaction . In this system, the alleles for susceptibility (r)
in the host and those for virulence (p) in the pathogen produce
specific compounds which interact with each other to produce the
susceptible response. A lack of interaction between the products of
r and p specifies resistant response of the host. This type of .gene-
for-gene relationship is found in heterotrophic pathogens (facultative
"parasites), e.g., Helminthosporium leaf blights (sugarcane, maize
etc.), 'Victoria blight of oats etc. The allele for resistance in the host
(R) produces a modified substance (a protein in the* case of sugar-
cane- Helminthosporium system) which is unable to recognise and
interact with the product of the allele for virulence in the pathogen.
Similarly, the allele for avirulence in the pathogen. (P) specifies *
■•■V
jik L
Breeding for Disease Resistance 387
slow spore production are generally involved. In this case, there is
some spore production, i.e., r>0. In some cases, the polygenic
inheritance may have a oligogenic component, the oligogenes acting
in an additive manner, e.g., bacterial blight resistance in cotton.
Presumably , polygenic resistance does not show pathotype-specificity
as against the oligogenic resistance. It may be seen that polygenic
resistance is almost the same as horizontal resistance.
Cytoplasmic Inheritance
In some cases, resistance is determined by cytoplasmic gene(s)
or plasma gene(s). For example, maize strains having the T male
sterile cytoplasm Ccms-T) in maize are extremely susceptible to
Helminthosporium leaf blight, while those having the normal or
non-T cytoplasm are resistant to this disease. Cases of cytoplasmic
inheritance of resistance are rare, and the above is the only good
example. b
SOURCES OF DISEASE RESISTANCE
Resistance to diseases may be obtained from four different
sources : (1) a known variety, (2) germplasm collection, (3) related
species and (4) mutations.
A Known Variety. Disease reactions of most of the cultivated varie-
ties are documented, and a breeder may find the resistance he needs
in a cultivated variety. Resistant plants were isolated from commer-
cial varieties in the cases of cabbage yellows in cabbage (B. oleracea),
curly-top resistance in sugar beets (B. vulgaris), mildew and leaf spot
resistance in alfalfa (M. sativa) etc. These provided the basis for
new resistant varieties of these crops.
Germplasm Collection. When resistance to a new disease or a new
pathotype of a disease is not known in a cultivated variety, germ-
plasm collections should be screened. There are numerous instances
where resistance to a disease was obtained from germplasm collec-
tions, e.g., resistances to net-blotch in barley (H. vidgare), resistance
to wilt in watermelon etc.
Related Species. Often resistance to a disease may not be present in
the varieties of the concerned crop species. In such cases, it would
be necessary to transfer resistance genes from related species through
interspecific hybridization (Chapter 25). Despite many problems in
such gene transfers, it has been successfully and extensively used in
many cases. A notable case is the resistance to stem, leaf and stripe
rusts of wheat ; a large number of resistance genes, e.g., Lr 9, Lr 19
Lr 22a, Lr 22b, Lr 24, Lr 25, Lr 26, Lr 28 and Lr 29 for leaf rust
resistance, and Sr 21, Sr 24, Sr 25, Sr 26, Sr 27, Sr 31, Sr 32, Sr 34
.and. Sr 35 for stem rust resistance have been transferred from related
species to wheat, and some of them have been widely used in crop
improvement programmes. Resistance to yellow mosaic has been
transferred from the wild species Abelmoschas manihot ; a yellow
mosaic resistant variety of bhindi ( Abelmoschas esculentus), Parbhani
Kranti, has resulted from this programme.
318
Plant Breeding : Principles and Methods
Mutations. Resistance to some diseases may be obtained through
mutations arising spontaneously, in some cases, or induced through
mutagen treatments, as often is the case. Resistance to Victoria
blight (Helmmthosporiuni victorias) in oats (A. sativa ) was induced
by irradiation with X-rays or thermal heutrons ; resistant mutants
were also isolated spontaneously in low frequencies. Some other
cases of induced mutations for resistance are : resistance to
stripe rust in wheat, crown rust in oats, mildew in barley, rust in
linseed (L. usitatissimum), and leaf spot (tikka) and stem rot in
peanuts (d. kypogaea).
METHODS OF BREEDING FOR DISEASE RESISTANCE
The methods of breeding for disease resistance are essentially
the same as those for other agronomic characters. The following
breeding methods have been commonly used : selection, introduc-
tion, mutation and hybridization.
Selection. Selection of resistant plants from a commercial variety is
the cheapest and the quickest method of developing a resistant
variety. The method has been useful in many cases in the past, but
it has only a limited usefulness at the present level of crop improve-
ment. Kufri Red potato is a disease resistant selection from
Darjeeling Red Round. Other examples include resistance to curlv-
tqp in sugar beets, to mildew and leaf spot in alfalfa, to cabbage
yellows in cabbage and to Periconia root rot in sorghum. Pusa
Sawani bhindi (A. esculentus) is a selection from a collection from
Bihar ; it is comparatively resistant to yellow mosaic under field
conditions. Cotton (G. hirsutun:) variety MCU 1 (Madras Combodia
Uganda 1) was selected from the variety Coimbatore 4 (CO 4) for
resistance to black-arm ; it has an acceptable level of resistance
under field conditions.
Introduction. Resistant varieties may be introduced for cultivation
in a new area. This offers a relatively simple and quick means of
obtaining resistant varieties. Introduction is quick, but has certain
limitations. Firstly, the introduced varieties may not perform well,
secondly, they may become susceptible in the new environment, and
thirdly, they may be susceptible to other diseases common in the
new area. For example, Kenya wheats (T. aestivum) introduced in
India were rust resistant but highly susceptible to loose smut. Simi-
larly, the American cotton (G. hirsutum) varieties were susceptible to
red blight-
introductions have served as a useful method of disease con-
trol. For example, Ridley wheat introduced from Australia has been
useful as a rust resistant variety. Early varieties of groundnut (A.
hypogaea) introduced from USA have been resistant to leaf spot or
tikka disease (Cercospora arachidicola). Kalyan Sona and Sonalika
wheat varieties originated from the segregating material introduced
from CIMMYT, Mexico, and were rust resistant. Introductions
also serve as sources of resistance in breeding programmes. For
Breeding for Disease Resistance
m
■example* African bajra (P. americqnum) introductions 'have been
used in developing downey mildew resistant male sterile lines (Tift
23A cytoplasm) for use in hybrid bajra production. This has been a
crucial development in the hybrid bajra programme since the original
male sterile lines Tift 23A and 23D S A were extremely susceptible to
downy mildew.
Mntatlon. .We have considered the usefulness of spontaneous at
-well as induced mutations in the breeding for disease resistance. The
methods of mutagenesis will be discussed in Chapter 23.
Hybridization. Hybridization is the most common method of breed-
ing for disease resistance. Hybridization serves two chief purposes :
(1) transfer of disease resistance from an agronomicallv undesirable
variety to a susceptible but otherwise desirable variety (by backcross
method), and (2) combining disease resistance and some other desir-
able characters of one variety with the superior characteristics of
another variety (by pedigree method). In the first case (backcross
method), the new variety is agrunomically the same as the sus-
ceptible variety, but is disease resistant. In the second case (pedigree
method), on the other hand, the new variety is expected to be
superior to both the parents in agronomic characteristics and at the
same time would be disease resistant.
In both the cases, one parent is selected for disease resistance ;
it should have a high intensity of resistance to as many races of the
pathogen as possible, and the resistance should be governed by few
oligogenes., When the resistant variety is unadapted and agronomi-
cally undesirable, backcross method is the obvious choice. But when
the resistant variety is well adapted and has some other desirable
features as well, the pedigree method of breeding is preferred.
Pedigree method . The pedigree method is quite suited for breeding
for horizontal or polygenic resistance as backcross method, in such
cases, is of limited value. Pedigree method for breeding of disease
resistance is not materially different from the method used for other
quantitative traits, e,g,, yield. In breeding for disease resistance,
artificial disease epidemics are generally produced to help in the selec-
tion for disease resistance (see later). Therefore, we shall not examine
the pedigree method of breeding for disease resistance in any detail.
A vast majority of disease resistant commercial varieties have beeh
developed through the pedigree method of breeding, e. g., Kalyafc
Sona, Sonalika, Malviya 12, Malviya 37 and many other wheat
varieties. G. hirsutum (American cotton) variety Laxmi, resistant to
red leaf blight, was developed through pedigree method from the
cross between susceptible parent Gadag 1 and the resistant parent
Coiiribatore 'Comt>ddia-2.
Backcross Method. The backcross method is useful in transferring
genes for resistance from a variety that is undesirable in agronomic
characteristics to a susceptible variety which is widely adapted arid
is agronomically highly desirable. The backcross programme would
3 90 Plant Breeding : Principles and Methods
differ depending upon the allelic relationship of the resistance gene,
Le whether it is recessive or dominant to the allele for suscepti-
bility. The backcross method as applicable to dominant (Fig. 15.2)
and to recessive (Fig. 15.3) genes for rust resistance is outlined in
chapter 15 ; it will not be discussed any further.
Generally, 5-6 backcrosses are made. A selection for the plant
type of the recurrent parent (the susceptible variety) during back-
crossing, particularly in BC 2 and BC S , is effective in making the
backross progeny resemble the recurrent parent rather rapidly. At
the end of the backcross programme, the progeny are selfed and
resistant plants are selected. Progenies derived from different resis-
tant plants that are identical in agronomic characteristics are usually
bulked to produce the new disease resistant variety. The new variety
would be almost identical to the recurrent parent, except for the
disease resistance ; hence extensive yield trials are usually not
required before its release for commercial cultivation.
TESTING FOR DISEASE RESISTANCE
Effective breeding for disease resistance depends upon the
identification of resistant plants from the susceptible ones with a
degree of certainty. The incidence of diseases under natural condi-
tions varies from year to year and their spread is generally uneven in
the field. Many susceptible plants may be classified as resistant in
such conditions. Therefore, it is often necessary to create artificial
epiphytotics or epidemics , that is a heavy incidence of a disease by
artificial means. The success in producing artificial epidemics
depends upon a precise knowledge of the pathogen and of the condi-
tions required for a heavy infection to occur (these conditions must
be provided). Further, a knowledge of pathotype differentiation in
the pathogen, and of the prevalent pathotypes is also essential fora
meaningful breeding programme. Therefore, most resistance breed-
ing programmes are based on cooperative efforts of plant breeders
and plant pathologists.
Disease resistance tests may be carried out in the field or in a
glass-house. Glass-house tests are more reliable since the favour-
able environment for disease development can be provided more
readily in a glass-house than in a field. An optimum temperature
and humidity are essential, though the optimum conditions for one
disease may vary considerably from that for another. The general
inoculation techniques used for the various catagories of pathogens
are briefly outlined below ; the exact details of the techniques are-
beyond the scope of this text.
Soil-Borne Diseases. Diseases, such as, wilts, root rot, damping off,
etc., are produced by fungi present in the soil. Generally, sick-plots
are created for testing resistance to such diseases. Sick-plots are
fields that have a sufficient inoculum load of the pathogen to infect
all the susceptible plants of the host population. Sick-plots may be
produced by mixing the soil from other sick-plots, by adding the
Breeding for , Disease Resistance
391
remains of diseased plants, or by adding inoculum produced in a
laboratory on host seeds, seedlings or on a nutrient medium* The
pathogen inoculum may be increased by growing a susceptible
variety for one or more years (the most commonly used technique)*
Glass-house tests may be carried out using the soil from a sick-plot.
Air-Borne Diseases. A vast majority of the diseases, e.g* 9 rusts,
smuts, mildews, blights, leaf spots etc., are produced by air-borne
fungi. Inoculation of such a disease may be done by dusting spores
from infected plants onto test plants, spraying a suspension of spores
or mycelium* injecting a spore suspension into individual plants or
leaves, or, in some cases, by planting rows of a highly susceptible
variety, i.e. 9 infector , to produce a large inoculum which is spread
by natural forces , e.g.> wind* In the case of wheat rusts, infectors
are commonly used ; Agra Local wheat is the common infector for
all the three rusts. In the case of air-borne fungi infecting ovary,
e.g, 9 loose smut of wheat and barely, spores are introduced in the
Sowers at the time of anihesis with the help of a forcep ora
hypodermic needle.
Seed-Borne Diseases. Some diseases are seed-borne, e.g. 9 some smuts,
wheat boot etc. In such cases, dry spores may be dusted on the
seeds before planting, e.g. 9 'in the case of sorghum smut. Alterna-
tively, the' seeds may be soaked in a suspension of poihogen ■ spores
under vacuum, e.g. 9 oat smut, covered smut of barley etc. Vhcuum
removes the- air from seeds and facilitates the entry of spores
underneath the husk.
Insect-Transmitted Diseases. Most of the viral diseases are transmitted
by insects ; such insects are known as vectors . Insect vectors include
aphids, leaf hoppers, etc. For inoculation of such diseases, insects
feeding on diseased plants are collected and transferred to healthy
plants. Alternatively, juice is extracted from the diseased plan!
tissues and rubbed on healthy plant tissues after . mechanical injury
has been inflicted to the latter. Mechanical injury is necessary to.
facilitate the entry of pathogens in the host tissues.
The foregoing is a very general description of the inoculation
methods for creating of artificial disease epidemics. The details, how-
ever, vary to a great deal from one pathogen to another. For disease
resistance breeding, a thorough knowledge of the pathogen is, there-
fore, a prerequisite.
DISEASE EPIDEMICS
An epidemic is a severe outbreak of disease beginning from a
low level of infection . Epidemics are of a common occurrence in the
case of air-borne fungi. Soil-borne fungi show epidemics when they
are introduced in a new locality, bat later on they show chronic levels
of infection. An epidemic progresses from a low infection level; often
a very low level of infection may initiate an epidemic. For example,,
an epidemic of potato leaf blight may be initiated by a single
1
Breeding jar m$eme Resistance
393
This minor disease then produces an epidemic due to a' widespead
cultivation of susceptible host genotypes. Victoria oat (A. sativa) 9
r bred for crown-rust resistance, had the dominant gene for suscepti-
bility to Helminthosporium blight, an unknown disease upto that
time. The male sterile T-cytoplasm in maize (Z. mays) was
susceptible to Helminthosporium leaf blight. The Tift 23A male
sterile inbred used as the female parent of the hybrid bajra (P. ameri -
canum) varieties in India was extremely susceptible to downy mildew
and ergot. In all the three cases cited above, minor diseases became
of severe epidemic proportions due to the high susceptibility of
prevalent host varieties.
Prevention of Epidemics .
The prevention of epidemics has been of great concern, and
several possible measures have been suggested for this purpose.
* These measures are, genetic diversity within varieties, between
varieties and in the breeding reserves, and the use of horizontal
resistance.
Genetic Diversity Employing Vertical Resistance. Strong genes for
vertical resistance may be employed in different ways to check
epidemics. Firstly , they may be used in alternate years so that the
virulent pathotype against any one of them does not evolve and does
not survive even if it did evolve. Secondly , they may be used for
planting in different geographical regions ; these regions would run
perpendicular to the direction of movement of the pathogen.
Thus each strong gene would act as a sieve for the pathotype virulent
on the succeeding one(s), A good example is provided by the wheat
stem rust resistance gene Sr6 in North America. There the rust mig-
^ rates northward; and before reaching spring wheats carrying the gene
Sr6, it passes through the winter wheat area lacking Sr6. Since the
pathotype virulent toward Sr 6, is a poor competitor, the winter
wheats act as a sieve for it. Sr6, as a result, has been the
most effective gene against stem rust in North America. However,
these two measures depend on a special condition that generally
cannot be ensured, that is, planting of specific varieties in specified
temporal or spatial order. Therefore, they cannot be relied upon.
The third measure involves the planting of different closely
situated fields to different varieties carrying different resistance genes.
This should reduce the severity of an epidemic, but has not been
practically tried. Th o fourth measure is the use of multiline varieties,
each pureline of the multiline carrying a different resistance gene
(Chapter 16). Multiline varieties may be expected to reduce the’
severity of an epidemic since only one or a few of the component
P-r lines may be expected to become susceptible at any one time.
Multilines have not been widely used commercially. Three Indian
wheat multiline varieties are KSML3, K ML 7406 and MIKSI1 ;
both are based on the popular wheat variety Kalyan Sona. However,
these multilines have not become popular among the farmers,
Breeding for Horizontal Resistance. The advantages sought by the
various measures outlined above using vertical resistance are all
394
Plant Breeding : Principles and Methods
provided for by horizontal resistance* In certain crop species, e.g. y
incase of potato for late blight, horizontal resistance. is the sole-
objective of breeders. There is a growing awareness for the value of
horizontal resistance and indications are that it will find greater,
applications in more and more crop species in the future. Some-
people believe that the ultimate answer to resistance breeding lies- in-
a wise use of horizontal resistance. Simmonds (1978) states that
*there is a substantial truth in the idea that we have the diseases to
which we breed susceptibility*.
SUMMARY
Disease is an abnormal condition produced by an organism, e.g., fungi-
bacteria, viruses, nematodes and insects. Diseases reduce yield by damaging
various plant parts or by reducing general vigour. There are generally several ■
pathotypes within a pathogen species. A paihotype is a strain of pathogen
virulent toward a specific resistance gene in the host (similar in meaning to
physiological race, but not the same). The pathogen produces new pathotypes
by sexual reproduction, heterocaryosis, parasexual reproduction and mutation.
Pathogenicity (ability of the pathogen to infect the host and, produce disease)-
is genetically controlled ; pathogenicity is generally recessive to noopathogeni-
city. ' Resistance of the host is the ability of the host to prevent the pathogen
from producing disease. Disease resistance is generally dominant to suscep-
. tibility, but in some cases resistance is recessive. The host- pathogen relation-
ship is described by the gene- for gene hypothesis of Ffor : for every resistance
gene present in the host, the pathogen has a gene for virulence ( ability to produce
disease), h susceptible reaction would result only if the resistance gene is
matched by the specific virulence gene in the pathogen. The hypothesis is
widely accepted, hut there are cases in which the “'pathogen is unable to match
the resistance gene present in the host.
A disease develops through four stages: contact, infection* establishment
and development. Susceptible hosts may be able to escape , disease due to un-
favourable environment for infection, earfiness, charge in the date of planting*
change in the site of planting, control of insect vectors and chemical disease-
control. Disease resistance may be immune reaction (no disease, r=-0)*
resistance (less disease, r>0 but less than ?) or tolerance (high disease
• incidence, but small loss in yield). Resistance is also classified as vertical and
horizontal Vertical resistance is oligogenic, pathctype-specjfic and generally
shows immunity. ^ Horizontal resistance is polygenic, pathotype non-specific
and acts by reducing r, i.e., r>0 but <1. The mechanism of disease resistance
tnay be mechanical, due to hypersensitivity, antibiosis or nutritional in
nature.
Genetically, disease resistance may show oligogenic, polygenic or cyto-
plasmic inheritance. Few oligogenes are strong, while many are weak (terms
refer to the competitive abilities of the pathotypes virulent against the resistance*
/ $©nc$). Oligogenes often show Vertifolia effect : severe epidemic development
when the pathotype virulent toward the resistance gene is evolved. For
breeding, disease resistance may be obtained from a known variety, from
germplasm collections, from related species or through mutation. The
methods of breeding for disease resistance include selection from a hetero-
jgeneous source population, introduction of a resistant variety, mutation and
.hybridization. Progeny from hybridization may be handled through backcross
©i pedigree method. An. important component of breeding for disease
resistance is testing for resistance by creating artificial epiphvtotics : Epiphyto-
tics are created in different ways for different diseases depending upon
,the nature of the pathogen and the environment necessary for the disease*
Breeding for Disease Resistance
395
Generally, diseases develop from a low infection level to a severe level ;
it is known as epidemic . An epidemic may start from a very low infection
level, e.g. t 1 in I0 7 plants. An epidemic results due to a narrow genetic base of
cultivated varieties, large areas planted to a single variety, introduction of a
new pathogen, and unknowingly^ breeding for susceptibility. Suggested
measures for preventing epidemics are based on vertical resistance or
horizontal resistance. Few strong oligcgenes may be used for planting in
alternate years in different geographical areas, in different fields or in a
mixture, e.g., a multiline variety. These measures ; : re expected to imitate
horizontal resistance, which appears to be the answer for preventing epidemics,
and signs are that in future it would become increasingly important in breeding
programmes.
QUESTIONS
V
t
1. Differentiate between : (i) horizontal and vertical resistance,
(ii) pathotypes and physiological races, {iii) disease resistance ao d
disease escape, (iv) virulence and avirulence, (v) immunity and
resistance, (vi) susceptibility and tolerance, (vii) resistance and
pathogenicity, and (vii) strong and weak resistance genes,
2. Briefly describe the mechanisms by which pathogens produce new
pathotypes.
3. The genetics of disease resistance is limited by the availability of new
genes for virulence, while that of pathogenicity is limited by new
genes for resistance. Comment with the help of suitable examples.
4. Define the following : (i) differential hosts, (ii) antibiosis,
(iii) hypersensitive reaction, (iv) vertifolia effect, (v) epidemics, and
(vi) sick-plots.
5. Write short notes on the following : (i) gene-for-gene relationship,
(ii) physiological races, (iii) Ideal differential hosts, (iv) artificial
epiphytotics (or epidemics), and (v) epidemics.
6. List the sources of disease resistance. Discuss any two of them in
detail with the help of suitable examples.
7. List the methods of breeding for. disease resistance. Describe the back-
cross method of breeding if the disease resistance is governed by a
recessive (or dominant) gene.
8. Effective breeding for disease resistance depends on a uniform spread'
of the disease in the breeding material. Discuss the above statement,
9. Describe the various methods for creating artificial epiphytotics.
10. Briefly describe the various mechanisms of disease resistance in plants.
11. Define epidemic. Discuss the causes of epidemics and the possible-
methods for their control.
Suggested Fen her Reading
Allard, R.W. 1960, Principles of Plant Breeding. John Wiley and Sons, Jnc, f
New York.
Brian, P.W. and Garrett, B.D. (eds.) 1972. A discussion on disease resistance
in plants. Proc. Royal Soc. London B, 181 : 213-351,
Browning, J.A. and Frey, K.J. 1969. Multiline cultivars as a means of disease
control. Ann. Rev. Phytopath. 7 : 355-382.
Day,P.R. 1974. Genetics of Host-Parasite Interaction. Freeman, San Francisco.
Flor, H.H. 1971. Current status of gene-for-gene concept. Ann. Rev. Phytopativ
9 : 275-296.
$ 9 $ Plant Breeding : Principles and Methods
Fwjy, K.J., Browning, LA., and Simons, M.D. 1973. Management of host
r^islimce geoes to control diseases. Z. PfiKrankh, Pffschutz. 80 : 160-
ffA&LAR, 2.R. 1976. Disease as a factor in plant evolution. Arm. "Rev. Phyto-
14 : 31-51.
Ingham, I»L, 1^76. Pbytoalexins and other natural products as factors in
•plant disease control. Bot. Rev. 38 : 343-424.
IkcmaMj, J. 1973. Disease resistance in higher plants. Fhytopath, Z. 78: 314-415.
Kiuick» RJ. and Malcqlmson, 3. 1973. Inheritance in potatoes of Held
resistance to late blight. Phys. Plant. Path. 3 : 121-131.
Kim, S.K. and Brewbaker, J.L. 1977. Inheritance of general resistance In
maize to Puccinia sorghi. Crop Sci. 17 : 456-461.
Lbppsk, E.B. 1977. Gene centres of plants as sources of disease resistance.
Ann. Rev. Phytopath. 8 : 323-344.
Nerson, R.R. (ed.) 1977. Breeding Plants for Disease Resistance. Pennsylvania
State University Press, University Park and London.
Parlevliet, J.E. 1981. Disease resistance in plants and its consequences for
plant breeding. In (ed. K.J. Frey) Plane Breeding II, pp. 309-364. The
Iowa State University Press, Ames, Iowa.
Roane, C.W. 1973. Trends in breeding for disease resistance in Crops. Ann.
Rev. Phytopath, i 1 : 463-486.
Robinson, R.A. 1971. Vertical resistance. Rev. Plant Pathol 50 : 233-239,
Robinson, R.A. 1973. Horizontal resistance. Rev. Plant Pathol. 52 : 483-
501.
1975. Gene-for-gene relationships in plant parasitic systems. Sci.
Progress, Oxford, 62 : 467-485.
Simmonds, N..W. 1979. Principles of Crop Improvement. Longman, London
and New York.
Van Der Plank, J.E. 1968. Disease Resistance in Plants. Academic Press,
New York and London.
CHAPTER 22
Breeding for Insect Resistance
All the crop species .are attacked by insects, but the degree of
damage to as well as the number of insect species attacking different
crop species vary considerably. For example, cotton is attacked by
more than 160 species of insects ; of these about a dozen are major
insect pests. An effective pest control measure is the basic require-
ment for a good cotton crop ; this is clear from the fact that 60%
of all the pesticides used in the world is applied to cotton. On the
other hand, crops like wheat and barley are attacked by a few insect
pests and none of these cause a serious damage.' It may be noted
that many insect pests have a rather narrow host range, i.e, f they
attack only a limited number of plant species. But many other
insects feed on several crop species, e.g., red cotton bug, Heliothfs
etc. Furthermore, an insect may move on to a new host species if
its normal host crop is not available.
Once. it. was hoped that chemical control measures will effec-
tively control, or even eliminate the insect pests. But the experience
with pesticides has shown that such a hope was entirely misplaced.
Extensive pesticide application (1) increases the cost of production
of crops, (2) reduces the population of natural enemies .(predators
and parasites) of insect pests, (3) leads to the development of pesti-
cide-resistant races of insects, and (4) pollutes the environment (the
most tragic and horrifying example is the Bhopal Gas Tragedy of
December, 1984). Most entomologists now speak of ‘ pest manage-
ment' in the place of ‘pest control *. Pest management involves, in addi-
tion to pesticide application, several divergent measures to minimise
the losses due to insect pests. Insect resistant crop varieties, where
available, form an important component of pest management schedules .
Insect resistance is the property of a variety of a host crop due to
which it is attacked by an insect pest to a significantly lower degree
than other varieties of the same host. From a practical view point,
an insect resistant variety produces relatively larger yields of good
quality than susceptible varieties with the same level of initial infesta-
tion (insect attack) under comparable environmental conditions . Thus,
resistance is a relative property and can be defined only in compari-'
son to other more susceptible varieties (Maxwell et al , 1972).
398
Plant Breeding : Principles and Methods
Insect pests may be grouped into two classes on the basis of
their mode of feeding : (I) sucking pests suck the cell sap, e.g.,
aphids, jassids, tfarips, white fly, mites etc., and (2) tissue feeders feed
on the various plant parts, e.g., stem or shoot borer, root borer,
fruit borer, leaf hoppers, weevils, beetles etc. Both the types of
insects reduce the quality as well as the quantity of crop yields. The
loss in quality of the produce is more frequent and often of a greater
concern than that of quantity per se. Insect infestation leads to one
or more of the following direct damages : (I) reduced plant growth
or stunting, (2) damage to leaf, stem, branch, flower buds, flower,
vegetative buds, fruits and seeds. (3) premature defoliation of leaves,
and even (4) wilting of plants. The degree of damage depends pri-
marily on the intensity of attack and the susceptibility of the host
strain. In addition, insects also caus,e indirect damage. Many
insects, e.g., aphids, mites, white fly etc., transmit plant viruses, i.e.,
serve as vectors of pathogens. Further, injuries caused by insects
make the plants more vulnerable to attacks by fungal and bacterial
pathogens.
The estimated global average loss due to insect pests in the
potential yields of all the crops is 14 per cent (Cramer, 1967). The
estimated losses in individual crops vary from 5 per cent in wheat to
26.7 percent in rice, and still more in crops like cotton and sugar-
cane. In spite of the widespread use of insecticides and other control
measures, the global average loss in most of the major crops exceeds
5 per cent. In India, losses due to insect pests range from 10 to 20
per cent. Losses due to insect pests are much more in tropical and
subtropical than in temperate regions since high temperature and
humidity promote insect activity and reproduction. In cases of
severe insect attacks, the yield losses may be upto 90 per cent. There
are several cases where a crop species was virtually eliminated due to
an insect pest. An excellent example is furnished by grape in France %
about 1 20 years ago its cultivation was virtually wiped out by
Phylloxera vertifoliae. The grape crop in France was saved by the
use of P. verri/o/tee-resistant root-stocks introduced from U.S.A.
Genetic Variability in Insect Pests
Strains of many insect pests, e.g., aphids, leaf and plant hoppers
of rice etc., differ in their ability to infest or attack varieties of the
same host species that differ in their .resistance to that pest. Such
strains of an Insect pest are referred to as biotypes ; the situation is
similar to the physiological differentiation in plant pathogens
(Chapter 21). However, biotype differentiation in insect pests
appears to be much less frequent than physiological differentiation in
the case of pathogens, and its consequences are generally not as
striking. The reasons for this are not very clear, but it may be due
to one or more of the following. Firstly, the number of offspring
produced in each generation by an insect pest is generally only a
Losses Dae. to Insects
Breeding for Insect Resistance 399
small fraction of the astronomical number produced by plant patho-
gens more particularly fungal pathogens. In addition, plant patho-
gens’ produce few to several hundred generations (bacteria and
viruses) in a single crop season as compared to one to few for most
insect pests. As a result, the chances of origin of genetic variation
through mutation and other mechanisms is much smaller in the case
of insect pests than in plant pathogens. Secondly, insect resistance
generally involves some morphological or some nonspecialized
biochemical/physiological factors of the host plant. Overcoming of
such mechanisms of resistance would require a change in the habit
or the physiology of the insect pest, which may be expected to be
governed bv more than one gene. As a result, biotype differentia-
tion of insect pests may be expected to be much less frequent than
physiological differentiation of plant pathogens since the latter
generally involves a single gene. Thirdly, biotype differentiation m
insect pests can be studied only when varieties of the host differing
for resistance genes are available. Relatively fewer cases of resis-
tance are known for insect pests than those for plant pathogens.
Fourthly, insect resistant varieties have generally not been cultivated
as widely as disease resistant ones. Therefore, the selection pressure
for the development of new biotypes of insect pests has not been as
great as that for new pathotypes in the case of plant pathogens. But
it may be pointed out that insect resistant varieties of many crop
species have been extensively grown for several years, but insect
biotypes capable of attacking them have not emerged so far. Similar
cases are also known for several plant diseases as well. Finally,
insect resistance has not been as widely investigated as disease resis-
tance as a consequence of the relatively smaller economic losses
caused by insects. This itself may be a sound reason for our limited
knowledge on the subject. More cases of bio-type differentiation in
insects are becoming known as they are being intensively investi-
gated.
An insect resistant variety may be attacked by a true resistance-
breaking biotype or merely by a more vigorous biotype. A true
resistance-breaking biotype is adapted to overcome a specific resis-
tance mechanism of the host plant. As a result, it is more success-
ful in attacking varieties which possess that specific resistance
mechanism than other host varieties. This gives rise to host-insect
pest interaction akin to host-pathogen interaction, e.g., biotypes of
aphids and varieties of raspberry, biotypes of leaf and plant hoppers
and varieties of rice etc. Sometimes, the biotype attacking a resis-
tant variety may not be a resistance-breaking biotype, but it may
simply be a highly vigorous biotype. Such a biotype attacks both
resistant and susceptible varieties of the host more successfully than
the preexisting biotype(s). Such a biotype lacks any specific adapta-
tion to overcome the resistance mechanism of the resistant -host
variety and it does not show host-insect pest interaction, e.g.,
cabbage aphid attacking rape varieties. In addition, a pest may be
differentiated into taxonomic races or it may show morphological
Plant Breeding • : Principles md Methods
m
variations which may or may not be associated with biotype differen-
tiation. Such variations are known in many insect pests, e.g,, wheat
Hessian fly, Similarly, a host variety resistant to one insect may be
attacked by another insect pest for which it was never resistant. For
example, some populations of potato cyst nematode have been
found to attack eel worm resistant potato varieties. These considera-
tions should be kept in mind while assessing the biotype differentia-
tion in an insect pest and the resistance of host varieties to various-
insects.
Mechanism of Insect Resistance
Insect resistance is grouped into four categories ; (1) nonpre-
ference, (2) antibiosis, (3) tolerance, and (4) avoidance. The first
three types of resistance were recognized by Painter (1951), while the
fourth, avoidance, was included subsequently. A given case of insect
resistance may exhibit one, more often, two or more of these mecha-
nisms. In some cases of resistance, nonpreference, antibiosis and.
tolerance are simultaneously involved, e.g., resistance of Brussel’s
sprouts to cabbage aphid. Such combinations of resistance mecha-
nisms are highly effective and are not easily overcome by the con-
cerned insect pest.
Nonpreference. Host Varieties exhibiting this type of resistance are
unattractive or unsuitable for colonization , oviposition or both by an
insect pest » This type of resistance is also termed as nonacceptance
and antixenosis. Nonaccepiance appears to be more appropriate
since in most known examples of this type of resistance the insects
will not accept a resistant host variety even if there is no alternative
source of food. There is a considerable variation in the degree of
nonpreference by insect pests. In some cases, the expression is so
strong that the insects walk-off the resistant plants, e.g , aphid resis-
tance in raspberry. In some cases, on the other hand, the insects
do not walk-off the resistant plants, but they feed for a compara-
tively shorter period and are generally restless on the resistant plants
as compared to on susceptible ones, e.g., aphid resistance in sugar-
beet (Russel, 1978). Nonpreference involves various morphological
and biochemical features of host plants.
Antibiosis. Antibiosis refers to an adverse effect of feeding on a resis-
tant host plant on the development and or reproduction of the insect
pest . In severe cases , it may even lead to the death of the insect pest.
Antibiosis may involve morphological, physiological or biochemical
features of the host plant ; some cases involve a combination of all
the three features.
Tolerance. Insect tolerance is similar to disease tolerance (Chapter
21).- An insect tolerant variety is attacked by the insect pest
to the jame degree as a susceptible variety. But at the same level of
infestation , a tolerant variety produces a larger yield than a susce-
ptible variety. In some cases, tolerant varieties show greater recovery
Breeding for Insect Resistance
401
>
\
1
than susceptible varieties from pest damage, e.g., in the case of
shoot borer in sorghum. In other cases, tolerance may be due to
the ability of host to suffer less damage by the pest, e.g., aphids
in sugarbeet and brassica, and greenbugs in cereals.
Avoidance. Pest avoidance is the same as disease escape and, as such,
it is not a case of true insect resistance . However, it is often as, or
even more, effective as true icsistance in protecting a crop from pest
damage. Most cases of pest avoidance result from the host plants
being at a much less susceptible developmental stage when the pest
population is at its peak. For example, early maturing cotton varie-
ties escape pink bollworra infestation which occurs late in the season.
In some cases, the crop may be grown in a season when the pest
population is very low, e.g., seed-plot technique in potato to avoid
aphid ..infestation.
Mature of Insect Resistance
Insect resistance may involve morphological physiological or
biochemical features of the host plant (Table 22.1).
Morphological Factors. A number of morphological factors, e.g.,.
hairiness, colour, thickness and toughness of tissues etc., are* known
to confer insect resistance.
1. Hairiness. Hairiness , of leaves is associated with resistance to*
many insect pests, e g., in cereals to cereal leaf beetle, in cotton to*,
jassids and in turnip to turnip aphid.
2. Colour of Plant. Plant colour may contribute to nonpreference in
' some cases. For example, red cabbage and red-leaved Brussel's
sprouts are less favoured than green varieties by butterflies and
certain other lepidoptera for oviposition. Similarly, bollworms.
prefer green cotton plants to red ones.
3. Thickness mi Toughness of Plant Tissues. Thick and tough plant
tissues present mechanical obstruction to feeding and oviposition
and thereby lead to nonpreference as well as antibiosis. For example*
thick leaf lamina in cotton contributes to jassid resistance, while*
solid stem leads to a resistance to wheat stem sawfly. Similarly*
thick and tough rind of cotton boll makes it difficult for the boll-
worm larvae to bore holes and enter the bolls.
4. Other Characters. Several other plant traits may contribute to
insect resistance. For example, Gossypium arbor eum varieties with
narrow-Jobed and leathery leaves are more resistant to jassids than
those with broad-lobed and succulent leaves. Cotton varieties with
longer pedicels are more resistant to bollworms than ^ those with
shorter ones ; a longer pedicel makes the movement of larvae from
one boll to another more difficult. Leaves of nectariless cotton varie-
ties are devoid of nector glands ; as a result, they are visited by?
insects less frequently than the normal plants.
Plant Breeding : Principles and Methods
Table 2-M. Nature of resistance to insect pests in some crop plants.
Mechanism of
resistance
Cause of resistance
Host crop Insect pest
Wheat . Hessian fly Hi to H s genes Antibiosis
Stem sawfly Solid stem Nonpreference and
antibiosis
Greenbugs High content of benzyl Antibiosis
alcohol
Barley Cereal leaf Hairiness and waxiness Tolerance and
beetle of leaves antibiosis
Rice Rice stem a. High silica content Antibiosis
borer b. Lignified stem Non preference
c. More schlerenchyma- Tolerance
tons tissues in stem
Brown plant a. Low asparagine con- Nonpreference
hopper tent of leaves
b. Red pericarp and Nonpreference
purple sigma
Maize European com DIMBOA content in Antibiosis
borer leaves
Maize stem High aspartic acid. Antibiosis
borer low nitrogen and
sugar contents
Corn ear Toughness of husk Nonpreference
worm
Cora leaf High DIMBOA in Nonpreference
aphid leaves arid antibiosis
Cotton Bollworms a. High gossypo! content Antibiosis
b. Smooth leaves Nonpreference
c. Nectar iless Nonpreference
d. Earlines Escape
Boll weevil a. Frego bract Nonpreference
b. Red plant body Nonpreference
c. Earliness Escape
Jassids Hairiness of leaves Nonpreference
tSugarbeet Aphids Low free sugar Nonpreferense and
antibiosis
Alfalfa Spotted alfalfa High saponin content Antibiosis
aphid and pea
aphid
Brassica Cabbage aphid Low sinigrin content Nonpreference and
antibiosis
Potato Aphid Sticky trichome exudat e Antibiosis
Physiological Factors. Some physiological factors, e.g., osmotic
concentration of ceil sap, various exudates etc., may be associated
with insect resistance. Leaf hairs of some Solarium species secrete
gummy exudates. Aphids and Colorado beetles get trapped in these
exudates and are unable to feed and reproduce. The exudates from
secondary irichomes of Medicago disciformis leaves have antibiotic
effects on alfalfa weevil; at lower concentrations it retards the
growth of weevil, while at higher concentrations it is lethal In
cotton, jassid resistance is associated with high osmotic concentra-
tion of the cell sap.
8r&&$gfor /meet Resistance
m
Biochemical Factors. $eyerjri biochemical factors are known to to
associated with insect resistance in many crops. It is believed that
biochemical factors are more important than morphological and
physiological factors in conferring nonpreference and antibiosis. ' A
well known example is the association between high concentration
of gossypol. a pheaolic compound, and resistance to several insect
pests in cotton. In rice, high silica content in shoots confers
resistance to shoot borer as it causes rapid wearing off of mandibular
mouth parts of the pest. Maize varieties resistant to European corn
borer show high concentration of DIMBOA (2, 4- dihydroxy, 7-methyl
2H-1, 4-benzoxoxazine, 3 (4H)-one), which is highly distasteful to
this pest. Similarly, resistance to greenbugs in wheat and barley is
associated with high concentration of benzyl alcohol. High saponin
•content of leaves and stem of alfalfa is associated with resistance to
spotted alfalfa aphid arid pea aphid. Lycopersicon esculentum has
an ethanol soluble compound which is highly toxic to tomato fruit
worm and tobacco flea beetle.
Soma biochemicals act as feeding stimuli for insects ; a defici-
ency of such compounds in the host may lead to insect resistance
{nonpreference). Asparagine acts as a feeding stimulus to brown
plant hopper (BPH) in rice. Varieties having relatively lower
concentrations of asparagine are more resistant to BPH than those
with higher concentrations. Similarly, simigrin acts as a stimulant
to cabbage aphid in brassicas, and a low concentration of this bio-
chemical in the leaves leads to a resistance to this pest.
■GENETICS OF INSECT RESISTANCE
Genetics of insect resistance is similar to that of disease resis-
tance. Insect resistance may be governed by (1) oiigogenes with
large individual effects, (2) polygenes with small additive effects or
(3) cytoplasmic genes (Table 22.2).
Oligogenic Resistance. In such cases, insect resistance is governed
by one or few major or oiigogenes, each gene having a large and
identifiable individual effect of resistance. In such cases, the resis-
tance may be conditioned by the dominant or the recessive allele of
a gene. The differences between resistant and susceptible plants
are generally large and clear-cut. In several cases, resistance is
governed by a single gene ( monogenic inheritance), e.g., in wheat to
green bugs, in raspberry to aphids, in rice to plant and leaf hoppers,
in cotton to jassids, and in apple to woolly aphis.
Monogenic resistance is simply inherited (3 : i ratio), is easily
transferred, involves a single feature of the host plant, is often
overcome by resistance-breaking biotypes, i.e., is less durable. But
some cases of monogenic resistance, e.g., jassid resistance in cotton.
Me quite durable as they have not been overcome by the pests even
after an extensive cultivation of the resistant varieties for substan-
tially long periods. Durable monogenic resistance mav be due to one
Plant Breeding : Principles and Methods
Table 22.2, Genetics of Insect resistance in some crop plants.
Mode of
Genetic
control
Crop
species
Expression
of resistance
Biotypes
known
Oligogenic Wheat
Rice
Grecnbugs id + m
Brown plant hopper 2D + 2 d
Gall midge 2D -rid
White- backed
plant hopper 3D+ ld
Green leaf hopper 6D+ id
Jassids ID + m
Aphids ID
Woolly aphis ID
Hessian fly 5D + 5d
Stem sawfly JD + ld
Greenbugs 2D
Corn borer NK
Pea aphid ID-Hd
Leaf beetle NK
Stem borer NK
Ear worm NK
Leaf aphid NK
Spotted aphid NK
Aphid NK
European corn Cytoplasmic
borer
Root aphid Cytoplasmic
Cotton
Raspberry
Apple
Wheat
Barley
Maize
Alfalfa
Polygenic Wheat
Rice
Maize
Alfalfa
Brassica
Cytoplasmic Maize
D— dominant, d«=recessive, modifier, NK=Not known, Resistance-
breaking biotypes known, -—Resistance-breaking biotypes not known.
or more of the following (1) cultivation of resistant varieties over
a relatively small area or time or both, (2) a lack of biotype differen-
tiation in pest species, and (3) the involvement of morphological
features of the host in resistance. Adaptation of the pest to a
morphological feature may necessitate a change in its habit which
may be expected to have a complex genetic control. However, it
may be pointed out that similar cases of durable resistance are
known for plant diseases as well. Modifying genes are known
to affect some genes governing insect resistance, e.g, 9 green bug
resistance in wheat, jassid resistance in cotton.
Where resistance is governed by two or more oligogenes, the'
genes may control a single character of the host plant or each
oligogene may govern a different trait. The latter situation Is more
common, and evolution of resistance breaking biotypes in such cases
is much less frequent than that in the cases governed by a single
major gene. Examples of such resistance are : in wheat to Hessian
and stem sawfly, in barley to green bug, in alfalfa to pea aphid etc.
An example of a highly durable resistance of this type is that asso-
ciated with solid stem in wheat to stem sawfly.
Polygenic Resistance. Polygenic insect resistance is governed by
several genes each with a small and usually cumulative effect. Such
of resistance (1) involve more than one feature of the host
Breeding for Insect Resistance 405
, , ar( . muc h more durable than the cases of oligogenic
plant, ( ) differences between resistant and susceptible
variation for the trait,
lufd (4) the transfer „°^ s S“e
adopt to more than one feature of the host plant,
rn ,„„ie«*!r Rocictance There are atleast two known cases of insect
SSano"ha*“« g ™m" by plasmagenes. These cases are,
resistance naiar Born faQrer . q majzc and resistance to root aphid
in lettuce E Itw likely that more cases of cytoplasmic resistance will
be discovered if extensive investigations were made.
SOURCES OF RESISTANCE
Resistance to a given inse'et pest may be found 1 in ( ) * “* j*
ted variety, (2) germplasm of the crop species, or (3) a related w
•species. a
Cultivated Variety. Resistance to many insect pests may be foun
% i ^rrVhSat ■fensA'sws "•& .»
good source’s of resistance 10 jassids. Utilization of rMistance torn
such a variety in a crop improvement programme presents te p
Sets aUt'f/presen, J an Vnomicaliy
Reactions of cultivated varieties of most of thec^rops f<> .
th S e 8C Jeq e uired Stance must' be made in cultivated varieties of 'the
concerned crop. It is worthwhile to introduce such varieties if they
are available in another country.
Germplasm Collection. .In case the desired "* “““ “.“onaUn S /m
in a cultivated variety, it should be searched in *= “ttonal an
world collections of the concerned crop species. J h f ^ e lasm
examples where the desired resistance was obtained from germplasm
“Eons. Screening of 2000 entries of apple
identification of 14 entries showing very high levels or rests*
the rosfapplc aphid, three entries entries immune to green apple
aph d and several lines resistant to apple sucker and l agf
Similarly, several cotton strains resistant to jassids have been identi
fiecTbv screening the germplasm. Utilization of insect resistance
from germplasm collections in breeding P ro ff*™™t : ’ e resistant lines
difficult than that from cultivated varieties nee the resistant nnes
mav not nossess good agronomic characteristics. ’ i t
KiSiTr mo*,e easily than .he resistance present m a relate
wild species. , : ■ . .
Related Wild Species. In many cases, ihe desired resis»ncc^iay^o
406
Plant Breeding : Principles and Methods
There are many cases where insect resistance has been transferred
from a related wild species to a crop species. For example, resis-
tance to both the species of potato nematodes has been transferred
from Solanum vernei to potato, resistance to root knot nematode
from a related wild species to tobacco, resistance for Rubus aphid is
found in a related wild species of raspberries, and jassid resistances
is known in the wild relatives of cotton, e.g , G. tomentosum ,
G. anomalum and G. armourianum .
BREEDING METHODS FOR INSECT RESISTANCE
The breeding methods for insect resistance are the same as
those for disease resistance. One of the following methods may be
used for this purpose : (!) introduction, (2) selection, and (3) hybri-
dization. The choice of breeding method in a given case depends
on the source of resistance, the mode of its inheritance and the
natural mode of pollination of the crop species.
Introduction. An introduced variety resistant to the concerned insect
may be released for cultivation if it performs well in the new envi-
ronment and is agronomicaliy desirable. Thus, it is the quickest and,
perhaps, the easiest method of developing an insect resistant variety.
But often the introduced variety may not perform well in the new
environment and it may be susceptible to the biotypes of the concern-
ed pest prevalent in the area or to new insect pests and/or diseases
of the area. There are many examples of introduction of insect
resistant varieties. .The most striking example is the introduction of
Phylloxera vertifoliae resistant grape root-stocks from U.S.A. into
France ; this saved the grape industry of France from virtual
extinction.
Selection. Insect resistant varieties may be present in an existing
variety of a crop. In such a case, selection for insect resistance is
practised to isolate an insect resistant variety. In self-pollinated
crops, mass selection or, more generally, pureline selection is practis-
ed for the purpose. Mass selection or, often, recurrent selection is
practised for developing an insect resistant variety in cross-pollinated
crops. Selection for insect resistance is generally more profitable in
cross-pollinated than in self-pollinated crops due to the existence of
large genetic variability in the former. An example of selection for
insect resistance is that for resistance to potato leaf hopper and
spotted alfalfa aphid in two broad-based germplasms of alfalfa ;
selection for resistance to both the pests was highly effective in both
these populations.
Hybridization. When the desired insect resistance is present in an
agronomicaliy inferior variety of the crop or in a related wild species,
hybridization is the only course of action for the breeder. Such
resistant species/strains are crossed with an agronomicaliy superior
: and adapted but insect pest susceptible variety of the crop to develop
a superior, well-adapted and insect resistant variety. In case the
resistant variety is expected to contribute to some of the agronomic
Breeding for Insect Resistance
407
characteristics also of the new variety, the F 2 and the subsequent
generations obtained from the cross are handled according to the
pedigree method ' This method is suitable for the resistance governed
by either olig ogenes or polygenes. . If the resistance is governed by
oligogenes, selection for resistance is effectively practised in the F*
generation. But in the case of polygenic insect resistance* (!) selec-
tion for resistance is delayed till the F 3 generation, and (2) a rela-
tively larger number of apparently resistant plants is selected. This is
necessary because the differences between resistant and susceptible
plants are not clear-cut, and there is a considerable effect of the
environment on the expression of resistance.
In many cases, the insect resistant variety may be agronomically
undesirable, or the gene for resistance may be present in a related
wild species. In such cases, backcross is the most suitable breeding
method. The resistant variety or the related species is used as the
nonrecurrent parent, while an agronomically superior, or high
yielding and well-adapted, but susceptible, variety is used as the
recurrent parent. The backcross method is commonly used when the
resistance is controlled by one or few major genes. But it may be
equally useful in cases of polygenic insect resistance as well provided
the heritability of the character is high. In general, 5-6 backcrosses
are necessary to recover the genotype of the recurrent parent. But in
the cases of interspecific transfer of resistance genes, a much larger
number of backcrosses may be needed. This is particularly so when
it is necessary to break the linkage between the resistance gene’ and
some undesirable genes. Interspecific gene transfers generally suffer
from a number of disadvantages discussed in some detail in a later
chapter (Chapter 25).
SCREENING TECHNIQUES
The most crucial and, perhaps, the most difficult task in
breeding for insect resistance is the identification of insect resistant
plants/lines during segregating generations. This requires an equal
level of initial infestation of all the plants in the population to ensure
that the plants scored as resistant are not escapes. It is often a tedius
and time consuming task to ensure an equal initial infestation of all
the plants, and in large populations it may not be practically possible.
Suitable techniques for screening for resistance to several insects have-
been developed and successfully used in breeding of insect resistant
varieties. The screening for insect resistance may be done either
(1) in the field or (2) in a glasshouse.^
Field Screening. Screening for insect resistance, particularly in
breeding programmes, is. usually done in the held since a much larger
number of plants can be screened in the field than in a glasshouse.
In addition, under fi eld conditions the plants are exposed to other
insect pests prevalent in the area ; this reduces the possibility of un-
intended selection for susceptibility to them. However, field tests are
carried out under uncontrolled conditions, and in many cases it may
408
Plant Breeding ; Principles and Methods
not be possible to ensure uniform initial' infestation of all the plants
in a population. As a result, experimental error of field tests is much
larger than that of laboratory tests.
There are many techniques .designed to promote uniform infes-
tation by an insect pest in the field. (1) The simplest technique
consists of interplanting a row of a known susceptible variety between
two rows of the plant material being tested. This technique is suitable
for air-dispersed insect pests only. (2) Each insect pest occurs much
more frequently in some areas of the country. Screening for resis-
tance to the concerned insect in such prone areas would ensure a
heavy natural infestation of the test plants. (3) In the case of soil
pests, the plant material being tested is planted in the soil known to
contain large populations of the concerned pest. Large populations
of soil pest can be maintained by regular planting of susceptible
varieties in the concerned field. This technique has been used for
screening oats and clovers for resistance to eelworm (4) Heavy
natural infestations of some pests may occur in a particular season.
For example, a much heavier infestation of rice stem borer occurs in
the off-season than in the main season crop since in the
off-season few alternate hosts are available to the female moths for
laying their eggs. Therefore, screening of rice for resistance to stem
borer is greatly facilitated in the off-season as compared to that in
the main season. (5) Finally, equal number of eggs or larvae may be
transferred by hand to each test plant. This ensures uniform initial
infestation, and is highly desirable wherever practically feasible For
example, rice plants are infested with stem borer by transferring egg
masses, containing about 60 eggs, to each test plant. The number of
larvae that hatch are counted and the resistant plants are identified
for further testing.
Glasshouse screening. A much smaller number of plants can be
screened in a glasshouse, if and where they exist, than in the field.
But the results from glasshouse tests are much more reliable than
those from field tests since both the environment and the initial
level of infestation are more or less uniform for all the plants- being
tested. Resistance to several pests has been screened in glasshouses.
For example, the larvae of rice stem borer are transferred, soon after
they hatch, to the test plants with the help of a soft brush. The
extent of damage on the test plants is then recorded and compared
with that on known susceptible control varieties infested in ‘the same
manner. In some cases, seedlings grown in seed boxes are infested
by the concerned pest ; resistant seedlings are identified and trans-
ferred to pots. After a period of recovery, the plants are reinfested
by the concerned pest and further selection is made for resistance.
This technique has been used for many .insects, e.g., screening of
alfalfa for resistance to spotted alfalfa aphid.
It may be pointed out that field and glasshouse screenings are
complementary to each other. The resistant plants/lines identified
through glasshouse screening should be evaluated for their resistance
under natural infestation, and vice versa .
Breeding for Insect Resistance
409
V
*
DURABILITY OF RESISTANCE
The term durability of resistance refers to the time required. for
-the appearance of a true resistance-breaking biotype of the concerned
insect pest. In case of durable resistance, the time required for
-emergence of such a biotype is quite long and in some cases such
biotypes have not been recorded so far. Examples of durable insect
resistance are : resistance to jassids in cotton, resistance to woolly
.aphid in apples, Hessian fly and stem sawfly resistances in wheat,
aphid resistance iri raspberry, resistance to spotted alfalfa aphid in
alfalfa etc. The emergence of new resistance- breaking biotvpes of
insect pests is far less frequent than that of new pathotypes of plant
pathogens. Even where new resistance-breaking biotypes of insect
pests have appeared, they have rarely led to the withdrawal of the
concerned resistant varieties from commercial cultivation ; this is
quite in contrast to the situation with disease resistance. The appear-
ance of new resistance-breaking biotypes is influenced by a variety of
factors, e.g., the insect species, the genetics of resistance, and the
features of host plant associated with resistance.
Insect Species. New biotypes of some insects appear more frequently
than those of others. For example, appearance of new biotypes is
relatively more frequent in nematodes and Hemipfera, particularly
aphids, than in moths and butterflies (Lepidoptera), beetles and
weevils (Coleo'ptera), and sawfiies (Hymenoptera).
Mode of Inheritance of Resistance. In general, resistance governed
by single major gene is less durable than those controlled by two or
more oligogenes or polygenes. However, some cases of resistance,
governed by a single major gene are veryf durable. On the other hand,
a few cases of polygenic resistance are as prone to new resistance-
breaking biotypes as monogenic resistance. For example, resistance-
breaking biotypes of cabbage aphid are known to overcome the
polygenic resistance of rape varieties in New Zealand and' England.
The longer durability of resistance governed by polygenes and that
by two or more oligogenes ■.stems'- '.from the fact that such 'cases-,
generally involve more than one feature of the host plant.
Morphological Characters. Insect resistance associated with morpho-
logical and/or anatomical characters of the host plant is ' generally
more durable than the other kinds of resistance. For example,
resistance to many pests is associated with hairiness of stem and
leaves ; hairiness interfers with oviposition by some insects, and
retards the development of larvae by impeding their movement.-,
Overcoming a resistance of this type would require a gross change ’
in the insect affecting its oviposition and larval movement. It is also
possible that the genes governing the morphological traits associated
with resistance in such cases may be closely linked with genes which
affect the development of the concerned pes* in other ways as well
(antibiosis).
There are some cases of durable resistance associated with bio-
chemical factors as well, e.g. f resistance of cotton to several pests
410
Plant Breeding : Principles and Methods-
associated with high gossypol content, resistance of maize to
European corn borer associated with high DIMBOA content, resis-
tance of rice to stem borer associated with high silica content, and
resistance of alfalfa to alfalfa aphid associated with high saponin,
content.
PREVENTION OF INSECT OUTBREAKS
Although true resistance- breaking biotypes of many insects
have been reported, resistant varieties have usually provided a good
and dependable control measure for them. The past experience
clearly indicates that the appearance of such biotypes is unlikely to,
reduce substantially the potential usefulness of resistant varieties in
the held before many years, and it is very unlikely that they will
lead to a complete breakdown of resistance over a wide area. There-
fore, breeding for insect resistance will continue to be a rewarding
objective in spite of the appearance of resistance-breaking biotypes.
The plant breeder, however, may adopt one or more of the following
approaches to reduce the chances of appearance of resistance break-
ing biotypes.
J. Different genes for resistance to a single insect pest may con-
trol different mechanisms, e.g., nonpreference, antibiosis and tole-
rance. Wherever such opportunities exist, the breeder should try to
combine two of more genes controlling different mechanisms into a
single variety. The resistance of such a variety will be much less
likely to be overcome by the insect pest than that due to a single
gene.
2. Whenever there is a choice of oligogenic and polygenic resistance,
the latter should be preferred as it is generally more durable. How-
ever, oligogenic resistance which is known to be durable is Far more
preferable to polygenic resistance since inheritance of and selection
for the former is far more simpler as compared to those for the
latter.
3. Some workers have suggested the use of multiline varieties.
Each component of a multiline should carry a separate gene for
resistance to a given insect pest. The usefulness of multilines would
depend on (1) the availability of different resistance . genes for an
insect, (2) nonavailability of a durable oligogenic or polygenic
resistance and (3) the extent of damage caused by the insect pest
to justify the effort and the expenditure in developing multiline
varieties.
4. Alternatively, varieties having different genes for resistance to
an insect pest may be planted in different years in a given area. In
this way, a variety having a given resistance gene is planted in the
area only after few to many years. As a result of such a rotation,
a given gene for resistance is not exposed to the pest for a long
enough time to permit the evolution of a resistance-breaking biotype.
However, it j$ generally not feasible to ensure a strict rotation over
Insect Resistance 4! I
a large area, and in many cases such a rotation may not be
necessary.
problems in breeding for insect resistance
Breeding for insect resistance suffers from several problems.
Some of the more important of these problems are outlined below.
1 Tn some cases, breeding for resistance to one insect leads to the
susceptibility to another pest. This may be because the feature of
host plant associated with resistance to one insect is associated with
susceptibility to another insect. For example, glabrous (hairless)
strains of cotton arc resistant to bollworms, but they are susceptible
to jassids.
2 In some cases, breeding for insect resistance reduces the quality
of produe or may even make it unfit for consumption. High go.ssy-
ool content is associated with resistance to bollworm in cotton.
Since aossvool is a highly toxic compound, a high gossypol content
f" “,hV Si SeSle as co.lon seed cake is an import. feed
for livestock, pigs and chicken.
O In manv cases, genes for insect resistance are available only in
the related wild species. Interspecific gene transfers present many
problems (Chapter 25), and often there is linkage- between the gene
for resistance and some undesirable genes. Vanet-es carrying genes
for insect resistance from related wild species are generally lower
yielding and their produce is often of inferior Quality.
4. Screening for insect resistance is the most critical and difficult
step in a breeding programme for insect resistance. This necessi-
tates a close coordination between the breeder and the entomolog 1 ^
In addition, pathologist, biochemist and physiologist have to be
doselv involved in an efficient breeding programme A close and
smooth collaboration between workers of such diverse disciplines is
an ideal rarely achieved in this country.
5. Breeding for insect resistance is a long-term programme. There-
fore adequate financing over a long period of time with a follow-up
action is essential. Often lack of funds leads to the interruption of
otherwise promising programmes.
ACHIEVEMENTS
Insect resistant varieties have been developed in man ^
in a number of countries of the world. The .most staking t « ^ P
of the contribution ol an insect resistant variety is. the intro “^ tl
of Phylloxera- resistant grape root-stocKS from USA ^into Fr<r .
In U S A 23 Hessian fly resistant wheat varieties have been de\e
JpYd These varietiesare successfully cultivated m the area of
Hessian fly incidence. Wheat variety ‘Rescue has solid stem and is
resistant to stem sawfly. This variety was first rfiT^and Canada-
Canada and is widely grown m those areas of U.S.A. and Canaoa
412
Plant Breeding : Principles and Methods
where this pest is a problem. Rescue has a lower yield and poorer
quality than some susceptible wheat varieties* Therefore, a rotation
•of two years Rescue-one year a high yielding susceptible variety keeps
the stem sawfiy population to an acceptable low level and allows a
■higher return than that from a continuous cultivation of Rescue.
A barley variety 4 WifF developed in U.S.A. is resistant to green
. 'bugs. Cultivation of this variety has reduced the green bug popu-
lation to Jess than 50 %. Alfalfa varieties ‘Cody’ ‘Moapa’, and ‘Zia*
•developed in U.S.A. are resistant to spotted alfalfa aphid. Oat
variety ‘Greta’, bred in Belgium, is resistant to stem eel worm. Cotton
varieties B 1007, SRTI, Khandwa 2; and DHY 286 evolved in India
are resistant to jassids and are successfully grown in the Central
India. Insect resistant varieties have been developed in many other
crops, e.g.y in raspberry for resistance to aphid, in lettuce for resis-
tance to root aphid, in grapes for resistance to Phylloxera , in apple
for resistance to woolly aphis etc.
BREEDING for resistance to parasitic weeds
Some weeds parasitise a number of economically important
crop species, and often cause a substantial reduction in their yields.
Jowar and bajra are parasitised by a weed called S trig a in some
.parts of India, e g Maharashtra, and Africa. It grows on the roots
of these crops and may lead to severe losses in yields in heavily
infested crops. Similarly, solanaceous crops like tobacco, tomato
etc. are infested by a parasitic weed called Orobanche, Orobanche
infestation of these crops is encountered in most pans of India. In
West Asia and the Mediterranean, Orobanche also infects winter-
sown chickpea (gram).
A convenient and relatively cheaper way of reducing the losses
in crop yields caused by such parasitic weeds is to develop resisiant
or tolerant varieties of their hosts. Their is considerable evidence to
suggest that breeding' for such resistant varieties is practically fea-
sible. A S7r/ga-resistant variety of jowar, SAR-I, has been developed
by ICRISAT, Hyderabad. This variety has shown high levels of
resistance to St rig a in the endemic areas of Maharashtra where
Striga infestation is a serious problem. Some other jowar lines have
shown high levels of resistance to Striga in certain pans of Africa.
Similarly, four bajra lines developed by ICRISAT show 90 per cent
less Striga incidence than the check variety. Eleven chickpea lines
resistant to Orobanche and 170 lines possessing tolerance to this
parasite have been identified by ICARDA, Syria. These ' lines are
the likely sources for resistance to Orobanche in chickpea breeding
programmes. .
These examples serve to illustrate vividly the point that resis-
tance to parasitic weeds may be located in the germplasm of the
host crops if an extensive and sustained search is conducted. Breed-
ing for resistance to such parasitic weeds may be an important breed-
ing objective in the areas where these weeds cause considerable
economic losses, and in few areas this may be as high , in priority as
that for a major disease or pest of the crop.
'Breeding for Insect Resistance
41 J
SUMMARY
Insect resistance is the ability of certain host strains to produce larger
yields of good quality than others at the >.anie level of initial infestation under
sjmilar environmental conditions. Insect attacks reduce yield as well a$ quality
of the procuce either by sucking the cell sap (aphids, jassids, thrips, mites etc.)
or by eating away plant tissues (borers* hoppers, beetles, weevils etc.?..
Resistance to insects results from (1) non preference (plants unattractive or
unsuitable for colonization or oviposition). (2) antibiosis (feeding on- olants
adversely affects the development or reproduction), (3) tolerance (ability of tne
host to produce more than other varieties, at the same level of infestation and
(4) avoidance (escape from infestation),' Resistance is associated with certain
(1) morphological (leaf and stern hairiness, solid stem, thickness and toughness
of plant tissues, colour of plant etc ), (2) physiological (exudates, osmotic
pressure of cell sap etc,), and (3) biochemical factors (gossypol, silica, benzyl
alcohol, tannin, saponin etc.)
Genetically, resistance may be controlled by (1) one or more major genes
( oligogenic ), (2) polygenes or (3) plasmagenes. Resistance may be completely or
partially dominant or recessive to susceptibility. The sources' lof resistance are
(1) cultivated varieties, (2) germplasm collections, and (3) related wild species.
The methods of breeding for insect resistance are 1 1) introduction. (2) selection,
and (3) hybridization (pedigree and backcross). Screening for insect resistance
may be done in the field or the glasshouse ; both the screening methods have
their merits and demerits
The durability of resistance depends mainly on the appearance of true
resistance-breaking biotypes. The appearance of resistance-breaking biotypes
depends on ( 1 ) the insect species, (2) the genetics of resistance, and (3) the
mechanism of resistance. Resistance of a variety miy be prolonged by
(1) bringing in a single variety two or more major genes governing different
mechanisms of resistance, (2) using polygenic resistance. (3> gene deployment,
and (4) using multilines. Many insect resistant varieties have been developed in
various crops in different countries which are successfully grown in the insect
prone areas.
Some parasitic weeds infest jo war, bajra, chickpea, tobacco, tomato and
several other crops. Some varieties of the host crops are resistant or tolerant to
these parasitic weeds. Resistant varieties of the host crop may be developed
through breeding in the endemic areas for these weeds.
QUESTIONS
1. Explain the various mechanisms of insect resistance, in crop plants
with the help of suitable examples.
2. Describe the different types of insect resistance with the help of
suitable examples.
3. Write short notes on the following : (/} genetic variability in insect
pests, ( ii ) inheritance of insect resistance , (Hi) true resistance-breaking
biotypes, (iv) durable resistance, (v) losses due to insect attack.
4. Differentiate between ; (0 antibiosis and antixenosis, (ii) resistant and
susceptible types, (Hi) oligogenic and polygenic resistance, Uf) insect
biotype and pathotype, (f) pest resistance and pest avoidance.
5. Describe the various sources of insect resistance, in crop plants,, and
discuss their relative usefulness in breeding for insect resistance.
6. Define durable resistance. Discuss the various factors affecting the
durability of resistance. Describe the approaches to increase the dura-
bility of insect resistance of a variety.
7 An unadopted variety has thi desired insect resistance which is
governed by a recessive gene. Describe the breeding method for
exploitation of this resistance. .
41 4
Plant Breeding : Principles ana methods
S. Describe briefly the various screening techniques for insect resistance*.
and discuss their merits and demerits.
9. ‘Genetic resistance is the most desirable and useful method of con-
trolling a pest*. Comment.
Suggested Further Reading
BEck, S.D. (1965). Resistance of plants to insects. Ann. Rev. Entom.
10, 207.
BEck, S.D. and Maxwell, F.G. (1973). In Biological Control (C. Huffakar, ed.).
Academic Press, New York.
Hedxn, P.A. (ed.) 1977. Insect resistance to pests. Ann. Chem> Sot. Symp. Ser.
62.
Khush. G.S. (1977). Disease and insect resistance in rice. Adv. Agron. 29 ;
265-341.
Maxwell, Jenkins, J.N. and Parrot, W.L. (1972). Resistance of plants
to insects. Adv. Agron 24 : 187.
Painter, R.H. (195!). Insect Resistance in Crop Plants. McMillan Co.,
New York.
Russell, G.E. (1978). Plant Breeding of Pest And Disease Resistance. Butter-
worths, London, Boston.
Waiss, A.C., Jr , Chan, G.G. and Elliger, C.A. (1977). Host Plant resistance
to insects. In Host Plant Resistance to Insects. An. Chem. Symp. Ser .
62 : 115-128.
CHAPTER 23
Mutations ia Crop Improvement
Mutation is a sudden heritable change in a characteristic of an
organism. This definition requires that the change in the charac-
teristic be heritable, but it does not state the genetic basis of the
heritable change. Cearly, a mutation (as defined above) may be the
result of a change in a gene, a change in chromosome(s) that involve;
several genes or a change in a plasmagene (genes present in the cyto
plasm, e.g., in chloroplast, mitochondria etc., which have circular
naked DNA as chromosomes). Mutations produced by changes in
the base sequences of genes (as a result of base pair transition or
transversion, deletion, duplication or inversion etc.) are known as
gene or point mutations . Gene mutations can be easily and clearly
shown by fine genetic analysis techniques available * with micro-
organisms. Some mutations may be produced by changes in chromo-
some structure, or even in chromosome number ; they are termed as
chromosomal mutations. Gross chromosomal changes, e.g., changes
in chromosome number, translocations, inversions, large deletions
and duplications, are detectable cytologically under the microscope.
But small deletions and duplications can rarely be detected, and
would be considered as gene mutations. This is particularly so in
higher organisms where the techniques of genetic analysis are not yet
as refined as those in the case of microorganisms. Thus what we refer
to as gene mutations in plants is likely to include a fair number of
small chromosomal changes. In clonal crops, mutations may even
include gross changes in chromosome structure, sometimes even in
number, unices cytological analyses are performed. Therefore , in this
chapter , the word mutation would be used without a reference to the
change in gene or chromosome ( but easily detectable chromosome
changes are not included) because in most of the cases the site of
change is not known : When the mutant character shows cytoplasmic
or extranuclear inheritance, it is known as cytoplasmic mutation .
Another term, bud mutation or somatic mutation , is used to denote
mutations occurring in buds or somatic tissues which are used for
propagation, e.g., in clonal crops.
HISTORICAL ACCOUNT
The term mutation was introduced by Hugo de Vries in 1900.
But mutations were known to occur in animals and plants much
416
riant Breeding : Principles and Methods'
before this time. For example, a short-legged sheep was discovered
by an English farmer in the 18th century ; this sheep was used to
establish a breed named Ancon. Mutagenic action of X-rays was dis-
' covered by Muller in 1927 on Drosophila , and in 1929 by Stadler in
barley (//. vulgare) and maize (Z. mays) ; Muller was awarded the
Nobel Prize in 1946 in recognition for this work. In 1946, Auerbach
and Robson showed that nitrogen mustards produced mutations in
Drosophila . Subsequently, a number of chemicals with mutagenic
action were described. Immediately after Muller's discovery in
1927, mutation breeding programmes were initiated in Sweden,
U.S.S.R. and Germany. The Swedish mutation breeding programme
was started in 1929 by Nilsson-Ehle and subsequently continued by
Ake Gustafsson and coworkers. The Swedish programme is perhaps
the most extensive covering several crop species, and much valuable
information has come out of the programme. Mutation breeding
attracted considerable attention during 1 950s and 1960s, and several
countries took up research projects in mutation breeding. In the
early days of mutation breeding, it was felt that - mutation breeding
would revolutionise plant breeding, and the breeder would be able to
create new genes at will. Indeed, this had been the case with the
other special techniques in plant breeding, e.g. 9 polyploidy, in vitro
techniques, etc. But as knowledge accumulated, it became clear that
although mutation breeding has a definite role to play, it is not the
magic wand which would create what you wanted and when you
wanted.
SPONTANEOUS AND INDUCED MUTATIONS
Mutations occur in natural populations (without any treatment
by man) at a low rate ; these are known as spontaneous mutations.
The frequency of spontaneous mutations is generally one in 10 lacs.
i.e.y i0~ 6 , but different genes may show considerably different muta-
tion rates. For example, R locus in maize mutates at a frequency of
4.92 X 10~h Su at 2.4 X 10~ 6 , while Wx appears to be highly stable.
Spontaneous mutation rates of genes may be considerably affected
by the genetic background ; some mutator genes may promote muta-
tion of other genes, e.g. y gene Dt (dotted) on chromosome 9 in maize
increases the mutation frequency of a (colourless aleurone) to A
(coloured a leurone), which is present on chromosome 3. Mutations
may be artificially induced by a treatment with certain physical or
chemical agents ; such mutations are known as induced mutations ,
and the agents used for producing them are termed as mutagens .
The 'utilization of induced mutations for crop improvement is known ,
as mutation breeding .
The available evidence indicates that induced mutations rarely
produce new alleles ; they produce alleles which are already known
to occur spontaneously or may be discovered if an extensive search
were made. It is reasonable to say that induced mutations are com-
parable to spontaneous mutations in their effects and in the
Mutations in Crop improvement
4 IT
variability they produce* But the induced mutations have, a great:
advantage over the spontaneous ones : they occur at a. relatively
higher frequency so that it is practical to work with them. Mutations
have certain general characteristics ; those that concern us the most
are summarised below,
L Mutations are generally recessive , but dominant mutations,
also occur,
2, Mutations are generally harmful to the organism . Most of the
■ mutations have deleterious effects s but a small proportion (ea.
0,1 per cent) of them are beneficial.
3, Mutations are random » i e., .they .may occur imuay gene. How-
ever,. some genes show higher mutation rates than others.
4. Mutations are recurrent , that is, ibe-same mutation may occur
again and again.
5. Induced mutations commonly show pleiotropy, often due to
mutations in closely linked genes.
EFFECTS OF MUTATION
Generally, mutations 'have harmful effects -on organisms.
Usually they reduce viability of the individuals that carry them.
Based on their effect cm viability! , mutations are classified into four
groups : lethal, sublethal, snbvital and vital.
; Lethal. Lethal mutations .kill each and every individual that .carries
them in the appropriate genotype. Dominant lethals, therefore,, pan-'
not be studied because they cannot survive even in the heterozygous
•state. Thus, of necessity, we have to consider only recessive lethals ;
that is, genes recessive’ in their lethal action. Recessive lethals would
kill the individuals that carry them in the homozygous state, e.g. t
albiea chlorophyll mutations.
Sublethal And Snbvital. Sublethal and subvital mutations reduce
viability, but they do not kill all the individuals carrying them.
..Sublethals. kill more than 50 per cent of the individuals, while sub-
' vitals kill much less than 50 per cent. A - vast mqjority of mutations
are lethals , , sublethals and subvitals ; .these -are of no value in emp
improvement , although some of them may be of academic interest.
Vital. Vital mutations do not reduce viability of the individuals
carrying them. Obviously , crop improvement can utilize only such
mutations , Vital mutations occur in a much lower frequency as com-
pared to the other three types .
MUTAGENS '
Agents used for the induction of mutations are known as muta-
gens. .Mutagens may he ..different kinds of .radiations (physical
418
Plant Breeding : Principles and Methods
mutagens) or certain chemicals ( chemical mutagens). A detailed dis-
cussion of various properties of the mutagens is beyond the scope of
this book. But some of the important properties of different muta-
gens will be summarised here. The different mutagens may be
grouped as follows.
A. Physical mutagens (all of them are various kinds of radiations)
1. Ionising radiations
(a) Particulate radiations, e g., a-rays (DI), {3-rays (SI),
fast neutrons* (DI), and thermal neutrons (DI).
(b) Nonparticulate radiations (electromagnetic radia-
tions), e.g.. X-rays* (SI), and r-rays* (SI).
2. Nonionising radiations, e.g., ultraviolet radiation (UV).
B. Chemical mutagens
1. Alkylating agents, e.g., sulphur mustards, nitrogen
mustards, epoxides, ethylene-i mines, (e.g., ethylene imine
or El)*, sulphates and sulphonates (e.g., ethylmethane
sulphonate or EMS*, methylmethanesul phonate or MMS),
diazoalkanes, nitroso compounds e.g., N'-methyl-N-
nitro-N-nitroso-guanidine or MNNG).
2. Acridine dyes, e.g., acriGavine, proflavine, acridine orange,
acridine yellow, ethidium bromide.
3. Base analogues, e.g., 5-bromouracil, 5-chlorouraciL
4. Others, e.g., nitrous acid, hydroxyl amine, sodium azide*.
I*denotes that these agents are commoly used in mutation breeding.
DI denotes densely ionising and, SI denotes sparsely ionising radia-
tions.]
Beta-Rays. Beta-rays are high energy electrons produced by the
decay of radioactive isotopes, e.g., 3 H, 82 P, SE S etc. High energy elec-
trons are showed down by positively charged molecules in tissues,
and they h.ave very little penetrating power as compared to X-rays.
Beta-rays transfer their energy to electrons of the atoms in their path
causing these electrons to fly away from their orbit leaving the
nucleus positively charged ; this is known as ionisation. When the
amount of energy transferred to an electron is not sufficient to cause
ionisation, the electron is pushed to an outer orbit representing a
higher level of energy, thus producing excitation. Electrons are easily
deflected by atoms in their path, hence they move in a zig-zag line.
After their energy is spent, electrons attach to an atom making it
negatively charged. Beta-rays may interact with the nuclei of atoms
to produce electromagnetic radiations similar to X-rays. Electrons
liberated as a result of ionisation also produce ionisation and excita-
tion, that is^they behave like beta-rays.
Alpha-Rays. The alpha particles making up alpha-rays have
two protrons and two neutrons each ; thus the alpha-particles have
double: pcrihve charge. Alpha-particles are produced by fission
of radioactive isotopes of heavier elements. Since they are heavy
Mutations in Crop Improvement
419
particles, they move in a straight line. Alpha-particles have a strong
attraction for electrons and pull them away from the nuclei of atoms
in their path. Alpha-rays produce both ionisation and excitation.
After losing energy, each alpha-particle captures two electrons and
produces an atom of Helium. As the a-particles move away from
their source, they slow down and produce dense ionisation. A!ph&-
particles are much less penetrating than neutrons and even beta-rays.
Fast md Thermal Neutrons. Fast Neutrons are produced in cyclo-
trons or atomic reactors as a result of radioactive decay of heavier
elements. The velocity of fast neutrons is reduced by graphite
or heavy water to generate thermal or slow neutrons. Neutrons
are uncharged particles, and are highly penetrating in biological
tissues. They are not repelled by nuclei of atoms, and move in a
straight line. They do not cause ionisation directly. Ionisation is
produced by (1) elastic scattering , in which nuclei of atoms are
kicked away by the neutron; these nuclei then cause ionisation,
and (2) production of gamma-rays as thermal neutrons are captured
by atomic nuclei, which then become unstable and give off gamma-
rays. Fast and thermal neutrons are densely ionising radiations.
X-rays and Gamma Rays. X-rays and gamma-rays are nonoar-
ticulate electromagnetic radiations with a wavelength of 1Q~ U
to 10~ 7 cm (Q.OONIGa or mu). These are high energy radia-
tions and consist of photons, /.<?., small packets of energy. The
physical properties and the biological effects of X-rays and. gamma-
rays are similar, but they differ in the source of their origin. X-rays
are produced by X-ray tubes, while gamma-rays are produced by
radioactive decay of certain elements, e.g. y radium, 14 C. 60 Co etc.
60 Co is the common source of gamma-rays used for biological
studies. X-rays are often referred teas hard (0.1-0,00lA) or soft
10- ! A) depending upon their wavelength. X-rays and gamma-rays
are highly penetrating and sparsely ionising. The electro-magnetic
radiations produce the following effects.
Photoelectric Effects. Low energy photons transfer all their energy to
individual electrons which are kicked off as high energy electrons
(e") from their orbit producing ionisation (Fig. 23.1). These high
energy e~ produce secondary ionisations which are of greater
significance than the primary ones.
Compton Scattering: A high energy photon transfers a part of its
energy in kicking away an e~ from its orbit producing ionisation. The
wavelength Of a photon becomes increasingly longer as it loses
energy in repeated ionisations (Fig. 23.1).
Pair Production . A high energy photon passing close to the nucleus'
of an atom may be completely absorbed accompanied with the
ejection of a high energy e~ and a high energy position (e + ); the high
energy e~ and e* produce ionisation and excitation (Fig. 23.1),
Ultraviolet Radiation. Ultraviolet rays have a wavelength of 100 to
3900 a (10-390 nm). UV is present in solar radiation and is produced
Plant Breeding 'Principles and -Methods:
426
Fig. 23.1/ Iod production by X-ray and gamma-ray photons. The large central
sdffd circle represents the nucleus of the atom* small, open circles or
the- periphery denote electrons (e X and the- particle e + -is a positron
The- atom pictured here is an oxygen atom.
jwtsmumsdm Crop Improvement
421
by mercury-vapour lamps or tubes, UV is a low energy radiation ;
it does not cause ionisation and has a very limited penetrating
capacity (usually limited to one or two cell layers), UV rays
generally produce dimers of thymine, uracil and, sometimes cytosine
present m the same strand of DNA. It also produces addition of a
molecule of water to the 5, 6 double bond of uracil and cytosine,
which promotes deamination of cytosine. The mutagenic action of
UV is most likely due to dimer formation and deamination. The
most effective wavelength of UV is 2,540 A since DNA bases show
the maximum, absorption at this wavelength.
UV is commonly' used in microorganisms since penetration
presents no problem in that system. But m higher organisms, poor
penetration of UV has limited its use to irradiation' of pollen grains
(in plants) and of small eggs (e g., in Drosophila )„ . In plants, pollen
grains may be itradiated'asd used for poHio&tiosh BtiMbo difficulty
in collecting large quantities of pollen grains in most of the crop
species, except in maize and similar crops, and the limited duration
of pollen liability ba*e- prevented the- use. of UN m oto$ improve-
mast.
MECHANISM OF ACTION OF RADIATIONS
Chemical effects of radiations are direct as wdi
In the case of direct effect, energy is transferred 1 to a molccnfe AMM*
ly by the radiation. Indirect effect is mediated by free radical for-
mation ; these radicals transfer their energy to other molecules. The
Indirect effect is particularly important in tie presence e£ water since
Ionised water molecules produce free radicals. Excitation has a very
low efficiency in. producing altered molecules,, but spm§£imes k may
cause dissociation of molecules. The primary effect of radio-
tions is due to ionisation ; the mechanism of ionisation differs to some
■extent from one radiation to the other . The chemiogl effects of
radiations may be summarised as under.
Formation, of Ioa Fairs. An electron is ejected, from an «$tom tQ,. pro-
duce a positively charged ion. The ejected er is capfiired by another
atom, lo produce a negatively charged Ion, Titus a pair of ions (one
positively and one negatively charged ion) i& produced, wbfcfc if
known as ion-pair (Fig 23.2). Oxygen (OJ ' II brie of the common
•electron acceptors. Biologically significant % events are considered to
be produced by ion-pairs.
Formation of Pro® Rmikafe, Free radicals sfewt VSt of
IQ~ 10 seconds. Free radicals are electrically neutral molecules with m
unpaired electron in their outer orbit, A fair portion of genetic effects
■of radiations are the results of the free radicals produced by irradia-
tion. Production of free radicals from is illiJMhfcted in
Fig, 20.3. Ao e~ is ejected from the.oxygeaatoiihQf Hg0, which, if
captured by one H 1 atom of another darter molecule producing an
ion-pair. In the presence of two other water molecules, the ion-pair
dissociates arid' produces two- free radicals.
422
Plant Breeding : Principles and Methods
IRRAOIATION
. a* + e° c* ejected from atom A
e~ captured by atom B
S
IRRADIATION
A*f B '
a* 4* s* The overall effect
(ION PAIRS)
pj g 23.2. Production of ion-pairs by high energy radiations.
H° and OH 8 are free radicals which are highly reactive. They
A n tin mio 4) and organic peroxides, which are highly
SS responsible for a good portion of the
biological effects of radiations.
HaO -
s IRRADIATION
— h 2 o* + qT e~ Is ejected from the O atom
HaO *
€
i
HjO
e~ is captured by one H atom
tN PRESENCE OP WATER
Hi©
Hi© HjO
t 6h° OH 0 is u free radical
h 2 O'
■ HaO HaO
M OH
H° is a free radical
Fig. 23.3. Production of free radicals by high energy radiations, using water
molecules as an example.
Mutations in Crop Improvement
*u$
Genetic effects of radiations result from their effect on
DMA. These effects include, change in a base, e,g,, deamination,
loss of a base, breaking of hydrogen bonds in DNA, single and
double strand breaks in DNA and cross-linking of strands. Pyri-
midines are more radiosensitive, /.<?., sensitive to radiation damage.
DNA present in the chromatin is much more protected than
naked DNA because of the protein coating. Deamination of
Free radicals are removed as follows,
—>Hs
GH 0 +OH°-»H*O 2
Presence of Oa increases the yield of H 2 O 2 ,
HOa° + HO/-»HsO * -fOa
'.HaOa is destroyed as fol-ows,
HgOjrf H®~ >HsO 4-OH°
Fig, 23.4. Production of H a O a from free radicals
bases and intrastrand dimerisation may lead to changed base pairing
producing changes in the base sequence of DNA, that is, gene
mutation.
MECHANISM OF ACTION OF CHEMICAL MUTAGENS
The mechanism of action of chemical mutagens varies to a
great extent front one group of chemical mutagens to another. It is
not possible to explain the action of all the classes of chemical
mutagens in this text. A broad general summary is presented instead.
The changes in DNA molecule produced by mutagens may be mu£a»
■genic or inactivating. Mutagenic changes do not prevent replication,
produce changes in one or more nucleotides and do not induce
chromosomal aberrations. They result from changes in hydrogen
bonding properties of bases or from mistakes in base pairing during
DNA replication. The events at the molecular level are either subs-
titution (transition -and transversion}, deletion or insertion of bases.
Substitution of bases generally has a less drastic effect than deletion
or insertion. Deletion and insertion often produce frame-shift muta-
tions since they usually change the sequence of bases in all the
codons that follow the point of deletion or insertion. Inactivating
alterations prevent DNA replication across the altered site, induce
chromosome breaks and chromosome mutations. The inactivating,
alterations include removal of bases, dimer formation, cross-linking
of the tw > DNA strands, and single or double strand breaks. Most
of the inactivating alterations of DNA are repaired by cellular
enzymatic repair mechanisms.
The mechanism of action, of alkylating agents is described in
some detail. The alkylating agents have one, two or more reactive
groups that react with DNA, hence they are known as mono-di-Or
424.
Plant Breeding : Principles and Methods
polyftiBCtionaJ, respectively. Polyfunction a!, sometimes even mono-
functional, agents cause extensive cross-linking of DNA, chromosome
breaks, chromosome mutations and gene mutations.. The action of
alkylating agents may be summarised as under.
1. Reaction with PO^-in ihe DNA P0 4 -siigar backbone producing
semistable triesters, This may cause backbone breaks.
2. Reaction with ring nitrogen , particularly in guanine with the N
at position 7, rarely with N m cytosine. Alkylation (reaction
with an alkyl group) of the ring N teads to the removal of a
base, i.e., depurination (since guanine is a purine) and may
lead to backbone breaks,
3 f Transition of G : C. to A : T may result due to mistakes in base
pairing following alkylation of guanine.
PROCEDURE FOR MUTATION BREEDING
Treating a biological material with a mutagen Jn order to
induce mutations is known as mutagenesis. Exposure of a biological
material to one of the radiations (X-rays, gramma-rays etc.) is
known as irradiation. When mutations are induced for crop improve-
ment,- the entire operation of the induction and isolation of mutants
is termed as mutation breeding . A mutation breeding programme
should be clearly planned and should be large enough with sufficient
facilities to permit an effective screening' of large populations. The
various steps involved in mutation breeding are briefly discussed
belbw.
Objectives of the Programme. A mutation breeding programme should
have well defined and clear-cut objectives . If the experimenter^ starts
a mutagenesis programme just with the hope that he will discover
something good, he is most likely wasting' his time and resources.
This is because the ratio of beneficial to' useless mutations is # very
. small (1 m 800 mutations, that is, about 0.1 per cent of mutations)*
and identifying desirable mutations from the undesirable ones is &
very difficult task indeed. Further, if a character governed by olig©«
genes is to. be improved, the procedure for the handling of treated
populations would-be different from that when a polygenic trait is to
be improved.
Selection- of the Variety for Mutagen Treatment. Generally , the
variety selected for mutagenesis should be the best variety available in
the crop . This is particularly so when polygenic traits are to, he
improved. It serves no purpose to isolate desirable mutants in a less
. adapted 1 inferior variety only to discover that the mutants have no
agricultural worth, or that the mutants have to be used in a hybri-
dization programme for transferring the mutant characteristics to- a
superior variety. In certain situations, however, it may be desirable
to isolate mutants in varieties other than the best one. For example,
act extensive search is being made for alternative dwarfing genes in
cereals, particularly in wheat and rice. (O. saliva). In this ease*, the
425
In sexually propagated crops, seeds are tfr
rt part. Pollen grains may. fee used, bat thr
tuse Cl) it is difficult to collect large quanti
aaLLaU*! i - * ui .« 111 I
lO'.ttie mtaaianoa of polka grams md egg cells. However the” Vnaatfl
mmt or whole plants requires special facilities (a gamma, garden) and
SffSJS p!aces o«*y. Chemical mutagf aea 4 St S
J3ta al^welL S °“ e WWtS have U3ed thsm with vegetative propa-
® as * Mutagen. Mutagen treatments reduce getminajiw
growth rate, vigour and fertility (pollen as well as ovalfflE^
considerahle kiihog of plants during the various ZZd!
mens atter mutagen treatment ; thus survival s, reduced ocnsiderabk-
“SMS T ce a high frss > a ™y * dSSJSSSi
changes and mitotic and meiotic irragulfrrities. Usually, the d&ma**
iQcrdases wita the mutagsn dose, bat it may not necessarily ha on
Phonal. An optimum dose is the me
mutat . lons causes the minimum killing. Many workers
j“ Cl a ^ ose c!os f to LDs 0 should be the optimum id*-. is
& ld““v 8 s,t J ? ,h *r ,d 1 tin 50 1“ «"< » f “«S" £i%
duals LD S0 vanes wnh the crop species and with the mutagen used
t^M mmaVy ex P enmeat » generally conducted to determine the
li able mutagen dose. In general, an overdose is likely to kill tan
7* Jims ' Whik m Underdose would P roduce J&
Dose of the mutagen may be varied by varying the intensify nr
the treatment time. In case of radiations, intensity may be varied hv
changing the radiation source or by changing the distance from the
.radiation source of the material being ijfaliated / foSUv t Je
4?on of mSgens mUtagenS ^ V£risd by chaa 3 ia » concentra-
®Ttog.t&e Mutagea Treatment. The selected plant part is esoosed
r e -lf7 t l mU ! agea ) dose - In case of irradiation! the SlamSS
Te w planted to raise M, plants from them (poltengK
are used for pollination). In case of chemical mutagens seeds **■*
exMsed^fo fh 3k d d f5w hours £o Vitiate metabolic activities
warn? to ^f C t r d i& 8gcn 2Ed thtm » running tS
water to. remove the mut^en present in- the seed* Tim treated s«2s
426
Plant Breeding : Principles and Methods
are immediately planted in the field to raise Ms generation. Mj is the
generation produced directly from the mutagen-treated plant parts
without a recourse to sexual or asexual reproduction. But when
pollen -grains are treated, the generation resulting from the seeds
produced by the treated pollen grains would be the Mi generation.
Mt, M*, M 4 etc. are the subsequent generations derived from Mi,
M 2 , Ms etc. plants through selling or clonal propagation.
Handling of The Motagee-Treafed Population. Treatment of seeds-
and vegetative propagules commonly produces chimeras. A chimera*
is an individual with one genotype in some of -its parts and another
genotype in the others. Shoot-tip meristem usually has two func-
tional layers : the outer layer ; giving rise to epidermis and a part of
leaf mesophyll, and the inner layer producing the rest of the plant
tissues including reproductive organs. When the entire outer or inner
layer is affected, the chimera is known as peridinal chimera (inner
periclinai or outer peridinal depending upon the layer affected), while
in. a sectorial chimera only a part of the inner or the outer layer is
affected (inner sectorial and outer sectorial respectively) (Fig. 23.5)
In sexually reproducing species, only the inner chimera (peri-
clinai or sectorial) will be transmitted to the next generation ; outer-
chimeras will not be recovered since this layer does not contribute-
to the production of gametes. In clonal crops, however, both outer
and inner chimeras can be utilized either as periclinai chimeras (outer
or inner) or by producing homogeneous individuals through sexual
reproduction (only if the inner layer is affected), tissue culture or
other horticultural manipulations, eg wounding etc., 'which induce-
production of adventitious shoot buds (both inner and outer chimeras-
utilized). Sectorial chimeras are unstable in clonal crops and have to-
be made periclinai through successive clonal propagation and’
selection for stability.
Mutations usually occur in small sectors of the meristem and
as a result only a part of the plant is affected. One or more, sexual-
or clonal generations coupled with selection are necessary to obtain
a stable mutant phenotype. Mutant alleles are generally recessive,
but some dominant mutations may also occur. In case of sexually
reproducing crops , mutations breeding utilizes both recessive and domi-
nant mutations and, in addition , excellent opportunities exist, for
mutation breeding for polygenic traits . Mutation breeding in clonal
crops primarily depends on dominant mutations ; recessive mutations
may also be utilized provided the done used for mutagen treatment
was heterozygous for the gene in question. . For example, if recessive-
mutant allele a is to be useful in a clonal crop, the clone has to have
the genotype Aa. Such situations are however rare ; more frequently,
the mutants useful in the improvement of clonal crops are dominant
mutation?? and may include changes in chromosome structure or
even number.
The following discussions are based on sexually reproducing
species, more particularly self-pollinated species. Since dominant
mutations are able to express themselves in. heterozygous state*
Mutations in Crop Improvement
outer
layer
INNER
LAYER
SHOOT-TIP i N ,ti AU
sectorial outer
Chimera
SECTORIAL INNER
CHIMERA
* 1
PERICUNAL INNER
CHIMERA
produced by mutagenesis. Tto
PERICUNAL outer
chimera
Fig. 23.5. Sectorial and pi
mutant ailefe is
428
Plant Breeding : Principles, and Methods
’ mutant plants are selected in- Mi. M 2 -and M 3 are raised and homo-
zygous mutants are selected. Selection for recessive mutations, how-
•ever, can be taken up in M* only, but the mutant allele will be
homozygous in the M g itself. Selection for polygenic traits is delayed
till M s generation, and is. based on individual plant progenies rather
than on individual plants. A generalised scheme for handling the
mutagen-treated populations for oligogenic and polygenic traits is
> outlined below.
Mutations in Oligogenic Traits. The handling procedure described
here (Fig. 23,6) is based on the selection for a recessive mutant allele*
FIRST
YEAR
Mi
00 O OQOO
jOOQ 0.000'!
loODOOOOf
loo'o ooooi
(0 Treated seeds are space-planted
(ii) .Seeds from individual plants
harvested separately
SECOND
‘ YEAR
Mt
I 1 1 f 1 1 (I) Individual plant progenies grown
(ii) Plants- from rows containing or
suspected to contain the mutant
allele harvested separately
THIRD'
YEAR
Mb
Nil!!
<£) Individual plant progenies grown
(ii) Superior mutant lines, harvested
I in "bulk, if they are homogeneous
(ill) I'm heterogeneous' progenies, in-
dividual plants may be selected
FOURTH M4
VEAR
| I 0) Preliminary yield trial with a
suitable- check
(ii) Superior lines selected
FlffTH-
years*"
(!) Replicated yield trial at several
t locations
(ii) Outstanding lines released', as a
new variety
•■Mnth-ow
tenth MgORMio
YEAR
Seed- mul£z$tfc&ttaa,- for distribution
among farmers
-FI*. 23.6. A generalised scheme for mutation breeding for an -oligogenic trait j
the mutant allele is recessive.
Mutations in drop Improvement
429
L5, ^7a!, hU °pXu”ed OT ,f r t ^ ’*'»
^hTSS EX 5} SOTSSK- ““ e »«SS-»S
to raise the M s progeny rows* “ P 1 e faarves,ed separately
Obs itTSSC "W'lT
are detected in M, became th* X • 0n ^ dist >nct mutations
Ptants. All the pl“ B to °" S ” 8 "
tions are harvested senarai.lv mV, • r containing new muta-
M,. If the mutant is' 5istio«! it S setaed fol hSnf'T” 1 ” ‘a
testtng. However, most of ihe muSf Ju
improvement. Only 1-3 oer cent of vr 1 De u ,? eiess for C /°P
have beneficial mutations. * * ma ^ bc cx P ec!ed to
1 M 3 Progeny rows from individual selected plants are grown in
M g . .Poor aad inferior mutant rows are eliminated. If the mutant
& r f? e -m eS * r T b ? mo S eaeou s> two or more 'M 3 progenies caatainine
Sl,Pl mUt3t T ^ aybs bu]ked - Mutant Ms rows are £Sd
in bulk for a preliminary yield trial in M«. siea
4. M 4 , A preliminary yield trial is conducted with a suitable checV
andpromising mutant lines are selected for replicated multilocation
f. Replicated multiiocatfon yield trials are conducted. The
outstanding line- may i be released as a new variety.
in Pph’genic Traits. There is.' considerable evidence that
“X 1 X 3r f 1 P duced in polygenic traits and that there is genetic
gam under selection. Mutagenesis does produce genetic variation in
polygenic traits ; mutant lines superior to the parent variety for poly-
genic traits including yield have been .isolated from mutagen-treated
populations through selection. A generalised scheme for the improve-
Slow (Fig P °23 8 ™ IC tr3,tS through rnutation deeding is outlined
1. Mi and M t . Mi and M 2 are grown in the same way as in the
case of oligogenic traits. In M 2 , vigorous, fertile and normal looking
plants tnardo ndt exhibit a mutant-phenotype are selected and their
seed: is. harvested --separately to raise 'Ms progeny rows.
2. Mz. Progeny rows' from individual selected plants are grown.
Careful observations are made on M s rows for small deviations in
phenotype from the parent variety. Inferior rows are discarded. Few
rows may be homogeneous and would be harvested in bulk. Selection
is done in M s rows showing segregation ;a majority of M, rows
would show segregation.
3. M a . Bulked seed from homogeneous M 3 rows may be planted in
a preliminary yield trial with a suitable check ; superior progenies
are selected for replicated multilocation yield trials. Individual plant
progenies from M 8 are critically observed. Progenies showing segre-
gation way subjected to selection only if they are promising
43D Plant Breeding: Principles and Methods
Superior homogeneous progenies are harvested' in bulk for prelimi-
nary yield tests in Ms.
(!) Mutagen«treated seeds space-
planted
(ii) Seeds from individual plants
harvested separately
first
YEAR
(i) Individual plant progenies grown
(ii) Fertile, vigorous, norm®! looking
plants harvested separately
(i) Individuj plant progenies from the
selected plants grown •
(ii) Superior plants selected ' from
superior progenies showing ssgrega*
tion
THIRD
«*Y£AR
(i) Individual plant progenies from
the selected plants grown
(ii) Superior homogeneous lines har-
vested in bulk
(iii) Segregating lines usually rejected
•■FOURTH M*
YEAR
(i) Preliminary yield trial with a suit*
able check
(ii) Superior lines selected
TIFTH
YEAR
(i) Replicated yield trial at several
locations
(ii) Outstanding line released as a
new variety
SIXTH-
TENTH
YEAR'S
Seed increase for distribution among
farmers
A ceneralised scheme for mutation breeding for polygenic traits.
431
Mutations in Crop Improvement
4. M 5 -M9. Preliminary yield trials and/or multilocation trials
are conducted depending upon the stage when the progenies
become homogeneous. Outstanding progenies may be released as new
varieties. •
Precautions. In mutation breeding programmes, the following two
precautions must be carefully observed. First , outcrossing with other
•varieties must be avoided in the Mi, since Mi plants show consider-
able male sterility. Therefore, prevention of outcrossing is necessary
even in seif-pofinated species. This may easily be attained by isola-
tion. Second precaution relates to avoiding of mechanical mixtures
just as in the case of purelines. If proper care is not taken, the
breeder may isolate mutations with surprising similarity to known
genotypes or varieties (but, in fact, these so called mutants are only
mechanical fixtures).
RECURRENT IRRADIATION
In case of recurrent irradiation, several successive generations
are subjected to irradiation treatment. Chemical mutagens may also
be used for recurrent treatment, but are not commonly used. For
treatment, a random sample of^eeds from Mi plants may be taken.
Alternatively, one or a few seeds from each plant may be used
for treatment. The dose of mutagen used for recurrent treatment is
generally lower than that used for single treatments. After 4-5
generations of recurrent irradiation, the material is handled as in the
case of single treatment.
It is expected that recurrent irradiation would accumulate
mutations in the population. But there is little evidence to suggest
that recurrent irradiation is a profitable scheme ; generally workers
have used. single treatments in preference to recurrent treatments.
.GAMMA-GARDEN
Gamma-garden is an area subjected to gammadr radiation.
This area is enclosed by thick-high wails to protect the plants and
animals outside this area from radiation damage. The purpose of a
. gamma-garden is to irradiate whole plants during different stages of
growth and for varying durations . The source of radiation is located
in the centre of the gamma-garden, which is usually circular in
outline. The intensity of radiation decreases as one moves away
from the source of radiation. The area around the radiation source
is divided into a number of concentric circles representing varying
intensities of radiation. Plants are placed in a suitable position in
. the gamma-garden, which depends upon the growth habit of the
species and the intensity of radiation required. The gamma-ray
source may also be used for the irradiation of seeds etc.
432
Plant Breeding t 'Ptindphss md .MwtksWi
The first »tnma-g«de 0 was * n ^- OB ® Island 'near
•■New York, US. A The first gamma-garden in India was wts ! ledoin
SSL . " Sr^?r, be \S
SSmUrcIteUtutc HARD in l«J * K^iTcfnm
g^'gr.iss? “«s
ffK riilMiafnSntan.^. 15? «5«£ of>Wo
SSSo CnriB. UK. .lumininm ? P»»in G b«Bnd '» » «
m TSflSd nonSi/b
imeT -'^he 1 aluminium capsule containing tie *Co pellets ivaiso
rid^ simultaneously . After irradiation the
5-5
ta g ««d most of them >ar«-aotna pperattoo.
DIRECTED MUTAGENESIS
It has been a long Cherished goal of the workers in mutation
breeding to control the direction of induced mutations, < ; e.. directed
mutagenesis Ideally, directed mutagenesis implies the induction of
•SrhSired mutation(s) at a high frequency But m practice,
1 ^ nnreciable increase (say 10 to 100 times) in the frequency
nfdeSdeSable mutations would be highly useful. Directed
mutagenesis was sought to be achieved by using different mutagens
Sv S well as in combination, and by varying the internal and/
or ^external environments before, during or after the mutagen
treatments. Many of these factors have been able to change- die
frequency of mutations and, to a small extent, the types of induced
imitations °as well. For example, irradiation of seeds having a
lower moisture content (3-5 per .cent) greatly increases mdiosensi-
ttVty and mutation frequency. In case of chemical mutagens
the duration of presoaking of seeds before .mutagen treatment
considerably affects the frequency of mutations recovered ; generally
a longer presoaking (opto a certain limit) produces a higher
frequency Of mutations, ^hese are, notable achievements ;but :the
frequency of desirable mutations- is- rarely increased, and it remains
approximately 1 out of -every -80ft mutations induced or so. Thus at
the present time, the type of mutations induced by mutagen/trcat-
ments is beyond experimental control. The breeder has. toaort out
what he wants from a lot of undesirable, although often fqacy,.
Mutations in Crop Improvement
433
I
APPLICATIONS OF MUTATION BREEDING
Mutation breeding has been used for improving both oligo-
• genic as well as polygenic characters. Mutagenesis has been used to
improve morphological and physiological characters, disease resis-
tance and quantitative characters including yielding ability. The
various applications of mutation breeding may be briefly summa-
rised as under.
!* Induction oj desirable mutant allcdes which may not be present
in the germplasm or that may be present hut may not be avail-
able to the breeder due to political or geographical reasons . To
some extent, mutation breeding relieves the complete depend-
ence of breeders on the natural germplasm. But it should be
remembered that mutation breeding cannot minimise the neces-
sity of germplasm collections ; it only serves as a useful supple-
ment to the available germplasm.
2. It is useful in improving specific characteristics of a well adapted
high yielding variety . This is particularly so in the case of
clonal crops due to their highly heterozygous nature ; in such
a case, mutagenesis is the only method available to improve
the specific characteristics of clones without changing the
genetic make up.
In self-pollinated species, mutagenesis is useful in improving
the specific characteristics of otherwise adapted and superior
varieties. However, in such species mutagenesis may not be
simpler or quicker than the standard backross procedure, if
'the characteristic is available in a variety. This is more so
*U -because the desirable mutations are often associated with
undesirable side effects due to other mutations, chromosomal
aberrations, sterility etc. One or few backcrosses with the
parent variety may be necessary to bring the desirable mutant
allele in an acceptable genetic background.
3. Mutagenesis has been successfully used to improve various
quantitative characters , including yield ' Several varieties have
been developed by this techique. However, there is no critical
comparison available to show that the same improvement
would not have been brought about by the conventional
hybridization programmes.
4. Fi hybrids from intervarietal crosses may be treated with muta-
gens in order to increase genetic variability by inducing muta-
tions and to facilitate recombination of linked genes . There Is
■some evidence that mutagen treatment of the Fi gives a rela-
tively wider spread (in frequency distribution) in the F 2 . Some
of the additional variability in F* is undoubtedly due to an
enhanced recombination between linked genes. But the method
has not been widely used in mutation breeding.
5. Irradiation of interspecific (distant) hybrids has been done to
produce translocations . This is done to transfer a chromosome
segment carrying a desirable gene from the alien chromosome
434
1 .
2 .
Plant Breeding Principles and Methods
to the chromosome of a cultivated species. This illustrates
another application of irradiations in crop improvement, hut
this does noj constitute mutation breeding.
LIMITATIONS OF MUTATION BREEDING
The experience with mutation breeding has brought out certain
limitations of the technique. These limitations are summarised as
under.
The frequency of desirable mutations is very low, about 0.1 per
cent of the total mutations. Therefore, large Mi , and ^se-
quent populations have to be grown and carefully studied.
This Involves considerable time, labour and other resources.
The breeder has to screen large populations to select desirable
mutations. Therefore, efficient, quick and inexpensive selec-
tion techniques are required to screen large populations.
Mutation breeding is more easily applied to such characters
where quick - screening techniques are available, e.g., disease
resistance. But in the case of characters where elaborate tests
are required, e.g., quality characteristics, mutation breeding is
virtually impractical. For this reason, mutation breeding has
been more successful with those characteristics where the
mutant phenotype is distinct and easily detectable.
Desirable mutations are commonly associated with undesirable
side effects due to other mutations, chromosomal aberrations
etc. The mutant line often has to be backcrossed to the
parent variety to remove these defects. This increases the time
requirement of mutation breeding programmes and mvqgves
additional labour, time and expenditure.
Most of the mutations are recessive. Detection of receive
mutations is almost impossible in clonal crops and is difficult
in polyploid species. Consequently, in polyploid species larger
populations have to be grown and larger doses of mutagen
have to be applied. Mutagenesis has been most commonly
applied to diploid species that reproduce sexually, particularly
in self-pollinated species.
ACHIEVEMENTS
More than 337 varieties have been produced as a result of
mutagenesis programmes in different countries of the world. Mutant
varieties have been developed in cereals, oilseeds, pulses, millets,
vegetables, fruit trees etc., but wheat, barley and rice account for
about 50% of mutant varieties in all the crops. ^ (Table 23.1). These
varieties represent improvements .in oligogenic as well as polygenic
characteristics. These crop varieties belong to diploid and polyploid
sexually reproducing species as well as to clonal crops. The first
variety developed from a mutagenesis programme was Primax White
Mustard (Brassfca hirta) released in 1950, followed by Regma II
Summer Rape ( B . campestris) in 1953. Both the varieties were
3.
4.
Mutations in Crop Improvement
Table 23.1.
Crop
4 35
Number of mutant varieties
Direct mutants
Mutants used in
hybridisation
Wheaf (T, aestivuni)
(T. iurgidum)
Barley
Rice
Rye
•Oat
. Maize
•Bajra
• Soybean
Groundnut
Rapeseed
Sunflower
Lie seed
Castor
Sesamum
■White mustard
Mungbean
Vigna av.gutaris
(azukfbean)
Rajma
Pea
Asrhar
Oram
Cowpea
Urd bean
Lupine
Cotton
Jute
Tobacco (N. tabacum )
Sugarcane
Potato -
Tomato
Onion
Green pepper
Lettuce
Currant
Forage crops
Fruit trees
Others
Cereals
24
?
29
44
3
4
Millets
3
r
Oilseeds
8
5
S
1
!
2
1
1
Pulses
3
1
5
6
3
5
2
2
2
Fiber Crops
5
5
Cash Crops
1
9
Vegetables
' I
4
2
3
2
1
Others
■ : : 4- ■ ' '
13
7
• 6
8
39
24
0
4
1
2
0
0
1
1
■0
2
0
5
2
0
0
0
0
4
0
1
4
0
0
1
0
1
0
0
1 .
0
0
Total
Total
30
15
68
68
3
8
9
7
5
1
2
3
1
3
1
10
8
3
5
' 2
2
6
5
6
5
9
1
5
2
4
2
1
■5
13
7
225
112
337
436 Plant Breeding % Principles and Methods
developed at the Svalof station of the Swedish Seed Association.
They represented a small but consistent increase in yield and oil
content over the parent variety and it took 10-13 years to develop
them. Further, there is some doubt that the improvements in these
two varieties were due to irradiation since both the crops (mustard
■and rape) show a high degree of cross-pollination. Why should the
cross-pollinated nature of the species raise doubts about the benefits
arising from mutagenesis ?
Fig. 23. S. Seeds of groundnut strain TG-19, a large pod
selection from a mutant X mutant cross
(Courtesy, Dr. C.R. Bhatia, BARC, Trombay).
Mutagenesis work in India started in 1930s on a small scale,
but received considerable attention during late 1950s and 1960s. A
number of crop varieties have been developed through mutagenesis,
e.g. y NP 836 wheat, Indore 2 cotton, JRO 514 and JRO 412 jute
(C. olitorius), Jagannath rice and Co 8152 and Co 8153 sugarcanes
(S. officinarum ) etc. (Table 23.2). NP 836 wheat is an awned mutant
from the awnless wheat variety NP 799. Although the variety
NP 836 has long been replaced by semidwarf wheat varieties, it
illustrates the usefulness of mutagenesis in changing a single charac-
teristic' of an otherwise desirable variety. Jagannath rice is a
gamma-ray induced semidwarf mutant from the tail cultivar T 141 ;
Jagannath has an improved resistance to lodging, higher yielding
ability is more responsive to fertilizer application than the parent
variety. Rice .variety CRM 13-32 4 was developed through muta-
genesis ,* it has reduced height and days to maturity, and inerted
Mutations in Crop Improvement
W"
437
Fig. 23.9* Seeds of Spanish improved (on the left) variety of groundnut
right) derived from the cross
T 3/ « J 7 * TG » 1 ls a law mutant of Spanish improved,
wfy*e 1G 17 wa'S developed by intercrossing two mutant ground-
nut trains {Courtesy, Dr, C.R. Bhatia, BARC, TrombayX
Table 23.2,
A list of some varieties
breeding. ’
developed in* India through mutation
Crop
Mutant variety
Parent variety Mutagen
'Chickpea
BOM 408
CPusa mutant 408)
G 130
BGM 413 (Pusa 413)
G 130
BGM 417 (Pusa 417)
BG 203
RSG: 2-
RS 10
RS 11,
rs m
Pea
Hans
Rabi cowpea
Arhar
' Moong
Urd
''Groundnut
lute
Wheat
N- 16
V 240
TV 1 (Trombay
Vishakha-l)
TAT 5
TAT 70
TAT 7
Fart Moong 2
CO '4' '
TAU-1
TG r: 17
TO I9A
TKJ 40
"'(Mabadey):
Sharbati; Sonora
Pusa Phalguni
Fusa Pb&lgtim
T '21
T-rays
T 21
y-rays
T-tays
S g
Y-rays .
CO i MMS (002%)
Ytfays
TG IxTG 17
(TG 1 is a mutant)
■ Y-rays
Sonora-64
Y-arays
438
Plant Breeding : Principles and Method 's
yield as compared to tm parent variety. ■ Often two low yielding
mutants may produce a high yielding progeny upon hybridization.
Jute variety- JRO 3690 is an example of this ; it was produced by
crossing two low yielding mutants of C. olitoriu :
Frabhavati variety of rice is an induced mutant- from^ the tall,,
scented variety Ambemobar Local which was treated with 0.2%
EMS. Frabhavati is semidwarf, nonlodglng and responsive to ferti-
lizer and nitrogen application. It yields 35 40 Q/ha as compared
25-30 Q/ha yield of the parent variety (Table 23.3). It has medium-
slender, translucent and scented grains.
Table 23.3. Characteristics of Frabhavati Bice.
Variety
Plant
height
(cm)
Days to
maturity
1000-gram
weight { g )
Straw
yield
(Q/ha)
Gram Lodging
yield score*
(Giha)
Frabhavati
(mutant)
75—80
115-120-
23.8
45.6
32.3
1)
Ambemohar
(parent)
120—830 ■
m
T
o
i— 1
23.0
64.5
28.3
4
♦Score.- 0—4 ; 0, no lodging ; 4, very -high degree of lodging.
Sugarcane variety Co 8152 is a gamma-ray induced mutant
from Co 527. . Co 8152 gives 40 per cent higher cane yield than the
parent variety, but it has an apparently undesirable feature. The
undesirable feature is that its lower leaves dry more quickly than in
the parent variety and in other sugarcane varieties. This has -limited
its popularity with the farmers, although the mutant variety is higher
yielding than the popular parent variety. This illustrates the undesir-
able side effects associated with desirable mutations. The mutant
variety Co 8152 has two chromosomes less than the parent variety
Co 527 . This is an example of a change in chromosome number produc-
ing a desirable mutant phenotype . Co 8153 is a gamma-ray induced
mutant from the sugarcane variety Co 775, which is highly suscep-
tible to smut. The mutant variety is highly resistant to smut and is as
productive as the parent variety. The chromosome number of this-
mutant variety is yet to be checked !
• Three mutant cotton varieties have been released for commer-
cial cultivation ; two of these (MCU 7 and MCU 10) were developed
through induced mutations, while one (DB-3-32) is a spontaneous
mutant MCU 7 was isolated from culture 1143 EE; it is eariy
maturing (145 days), has medium staple length and is. suitable for
cultivation in rice fallow areas of Tamil Nadu. MCU 10 is a mutant
from MCU 4 induced by 30 kR gamma-rays ; it performs better
than the parent variety (MUC 4) under rainfed conditions. DB-3-12'
is a spontaneous mutant from the G. herbaceum variety Western 1 ;
it has a better plant type and matures 20 days earlier than the
arent variety.
Mutations in Crop Improvement 439
Tehte 23.4. Proportion of the v*rseties developed through treatments with
physical and chemical mutagen* in sexually and asexaaily propa-
gated crops.
Mutagenic treatment
Sexually
AsexuaUy
propagated
propagated
crops
crops
Physical mutagens
89.2
98,8
Chemical mutagens
: 9.f :
- 0.8
Physical and chemical
mutagens combined
i 2
' 0 A
According to an estimate, about 94% of the mutant varieties
developed till 1982 were developed following treatments with physical
mutagens, about 5% through chemical mutagenesis and the remain-
ing 1 % through a combined treatment, with both physical mi
chemical mutagens. Further, a much higher proportion of the mutant
varieties owe their origins to physical mutagens in asexuaily propa-
gated crops than in sexually reproducing ones (Table '23.4). This
may not reflect a greater effectiveness of physical over chemical
mutagens ; it may simply be related to the easier availability of the
former resulting in its much more extensive application.
SUMMARY
Mutations are sudden heritable changes in the characteristics of organisms.
Mutation* may be chromosomal, cytoplasmic or gene mutations (point muta-
tions). Mutations occur spontaneously at a' low frequency, ca. 10~ A are usually
recessive, deleterious m effect, random and recurrent in occurrence. Mutations
can be induced at relatively higher frequencies .by treatment with certain
radiations (physical mutagens) or chemicals (chemical mutagens). Mutagenic
action of X-rays was first discovered by Muller In 1927, and that of nitrogen
mustards by Auerbach and Robson In 1946. Based on their effect on survival,
mutations are classified into four groups : lethal, sublethal, subvital and vital.
Mutation. breeding utilizes vital mutations only.
Its mutation breeding, gamma-rays and X-rays are the most commonly
used physical mutagens, while EMS, El and sodium azide are the most
commonly used chemical mutagens. X-rays and gamma-rays act by causing
Ionisation directly in the DNA or in the medium which produces free radicals.
Free radicals are highly reactive and produce RaOg and organic peroxides which
act on DNA. EMS and El act primarily by chemical interaction with DMA,
e.g. t alkylation of N at 9 position in guanine.
• A mutation breeding programme should .have a well defined and clear-
cut objective, and the variety to b© Irradiated should be carefully chosen.
Seeds, pollen grains, buds or cuttings may be used for mutagen treatment;
seeds are more commonly treated due to ..ease in handling. The dose of muta-
gen should be decided by experimentation';, a dose close to LD 50 is generally
.preferred. Subsequent handling of the-treated population in M fi and later
generations depends upon' the -mode of reproduction of the crop species, and
on the ol go genic or polygenic nature of the mutations. About 9-10. generations
(years) may be required in developing a mutant variety. Recurrent mutation
treatment^- c^y be-fivem and whole plants may be irradiated at various stages
of development' at a gamma-garden. Gamma-gardens were established at Bose
Institute '-Calcutta, IARI, New Delhi and BARC, Trombay.
Control qo the direction of mutation ( directed mutagenesis) is not feasible
at present. '..Mutation breeding has been used' to improve both oligogenic and
440
Plant Breeding : Principles, and Method
polygenic traits ; more than 300 varieties have been developed through muta-
genesis in various countries.-- Mutation breeding is the only technique available
for changing specific characters in clonal crops. Application of mutation breed-
ing is limited by low frequency of desirable mutations, nonavailability of quick
and efficient screening techniques, association of undesirable side effects, and
recessiveness of most of the desirable mutations, particularly in polyploid and
clonal crops.
QUESTIONS
1. Compare and differentiate between the following : (i) spontaneous
and induced mutations, (ii) chromosomal and gene mutations,
(Isi) mutation and mutagen, (iv) UV and gamma radiations,
(v) physical and chemical mutagens, (vi) oligogenic and polygenic
mutations, (vii) cytoplasmic mutations and bud mutations,
(vfii) periclinal and sectorial chimeras, (ix) particulate and non-
particulate radiations, (x) ionising and non ionising radiations, and
(xi* mutagenesis and mutation breeding.
2. Define the following : mutator genes, mutation breeding, ionisation,
excitation, depurination, irradiation, Ms generation, optimum mutagen
dose and LD so .
3. Define mutation. Briefly describe the characteristics of mutations and
their effects on survival of organisms.
4. Define mutagen. List the different mutagens and briefly describe the
properties and the mechanisms of action of either physical or chemical
mutagens.
5. Write short notes oa the following : (i) ionisation, (ii) free radicals,
(m) alkylating agents, (iv) gamma-garden, (v) recurrent irradiation,
(vi) directed mutagenesis, (vii) UV rays* (viii) mutagenic DMA
■ alterations, and (ix) chimeras*
6. Discuss the physical and chemical effects of radiations.
7. Outline a generalised procedure for mutation breeding with specie!
reference to oligogenic characters.
8. Discuss the applications of mutation breeding in crop improvement.
9. Describe the achievements of mutation breeding and discuss the limi-
tations of this approach of plant breeding.
Suggested Farther Readings
Allard, R.W. I960. Principles of Plant Breeding. John Wiley and Sons, Inc,*
New York.
Auerbach* C„ 1976. Mutation Research. Chapman and Hall, London.
Elliot, F.C 1958. Plant Breeding apd Cytogenetics. McGraw Hill Book Co.,
Inc., New York,
.A., Hagbero, A., Persson,, G. and Wxklund, K. 1971. Induced
mutations ' and barky improvement. Tfeeor. Appl. Genet. 41 ; 239-
. ■ 248.
IAEA, 1964.' 7 he. use of Induced in Plant Breeding. Inter oaf ion
Atomic Pn<?rffV Vienna
441
Mutations in Crop Improvement
IAEA, 1970. Manual on Mutation Breeding. International Atomic Energy
Agency, Vienna.
IAEA, 1973. Induced Mutations in Vegeta tively Propagated Plants. Inter-
national Atomic Energy Agency, Vienna.
IAEA, 1974 Polyploidy and Induced Mutations in Plant Breeding. Inter-
national Atomic Energy Agency, Vienna.
Prasad, A.B. (ed.) 1986. Mutagenesis ; Basic ■ and Applied. (Partmclariy,
Micke, A., pp. 75-80, Gottschalk, W„ pp. 81-96}, Print Hous: (India),
Lucknow.
Redei, G.P. 1974 Economy in mutation experiments, Z. PjSanzen. Zucht. 73 :
87-96. ' *
-Simmonds, N.W, 1979. Principles of Crop Improvement. Longman, London
and New York.
her of a species remai
are very precise as a
some numbers different irom me normal somatic cnromos
Bomber of tie concerned species. Changes in chromosome tmi
(some types) have contributed greatly to crop evolution, and (a]
types) are of much use in plant breeding. In this chapter
shall discuss in some detail the types of changes in chromes
number, their cliaracterstics, production and applications in
Improvement.
TYPES OF CHANGES IN CHROMOSOME NUMBER
The somatic chromosome number of any species, whether diploid
or polyploid, is designated as 2 n 9 and the chromosome number of
gametes is denoted as n. An individual carrying the gametic
chromosome number, n, is knwon as haploid . A monoploid, on the
other hand, has the basic chrome some number, • x In a diploid
species, n=x. ; one x constitutes a ; genome m chromosome comple-
ment. The different chromosomes of a single 'genome 'are distinct from
each other in morphology and! or gene conMMrmd homology ; members
of a single genome do not show a tendency of poking with -each other t ;
Thus a diploid species has two, a tribloid has 3 and a tetraploid has
4 genomes and so on.
The terminology of heteroploidy is summarised in Table 24. L
Individuals carrying chromosome numbers other than the diploid
(2x and not 2 n) number are known as heteroploids, and the situation
is known as heteroploidy. The change in chromosome number may
involve one or a few chromosomes of the genome ; this is known as
aneuploidy . The aneuploid changes are determined in relation to the
somatic chromosome number {2m and not 2x) of the species in question.
Therefore, the terminology for aneuploid individuals arising front
Polyploidy in Plant Breeding
Table 24.1. A summary of the terms used to describe heteroploidy
(var i ation in chromosome number)
Symbol*
Term-
Heceroploid
A* A ae opioid
.Nulllsomic
' Monosomic
Doub’e
monosomic.
. Trisomic
Double
trisomic
Tetrasomic
Eupfoid
Haploid
■L Mon opioid
2, Autopoly-
ploid
Auto triple id
Autotetraploid
Autopentapioid
Autohexaploid
Autooctapjoid
3., Allopolyploid
B.
Type of change
Change from 2x
Oae or a few chromosomes extra or
missing from In
One chromosome pair missing
One chromosome missing
One chromosome from each of two
different chromosome pairs missing
One chromosome extra
One chromosome- from each of two
different chromosome pairs extra
One chromosome pair extra
Number of genomes more than two
Gametic chromosome complement
One genome
2/i ± few
2b-1-
2n 4“ I
2/i+f -
2«+2
Allotetraplbid
AHohexaploid
Allooctaploid
£2x i + 2.r c )**
' Genomes identical with each other
Three genomes
Four genomes
Five genomes
Six genomes
Eight genomes
Two or. more distinct genomes
(generally each genome has two
copies)
Two distinct genomes
Three distinct genomes (2x l +2a\j + 2x0*'*
Four distinct genomes {2x\ + 2x*+ 2x s + 2,v*)**
*-2ri ~ Somatic chromosome number and ,/*«ga meric chromosome
number of the species, whether diploid or polyploid,
x— The basic chromosome number or genomic number.
xu xt> xu x^Disiinct genomes from different species, '
**fn general,' this condition occurs ; other situations may also occur.
diploid and polyploid species is the same. Heteroploidy that involves
one or more complete genomes is known as euploidy . By definition,
therefore, the chromosome number of euploids is an exact multiple
of the basic chromosome number of the species, while that of aneu-
ploids is not. : .V 7 7 • , . 7 . . ,
. ■ Aneupioid individuals from which one chromosome pair is
missing (In -1) are termed as mdlisomics , while those lacking a
single chromosome (In -I) are known as monosomies. A doable
' monosomic individual has two chromosomes missing, but the two
chromosomes belong to two different chromosome pairs (In - 1 — 1).
An individual having one e v tra chromosome (2/7 4-1) is known as a
trisomic , and that having two extra chromosomes each belonging to
a different chromosome pair is double trisomic (2/?4-I-rl). When
an individual has an extra pair of chromosomes, it is known as
tetrasomic (2n+2). The detailed terminology describing aneuploidy
is very complex, and is beyond the scope of this book. The same is
true about the various features of and the facts accumulated about
444
Plant Breeding : Principles and Methods
aneuploidy as well as euploidy. Here, I shall limit the discussion to
those aspects that concern a breeder the most. The breeder is gener-
ally concerned, with monosomies and trisomics, and in some situations
with nullisomics and tetrasomics.
In euploids, the chromosome number is an exact multiple of
the basic or genomic number. Euploidy is more commonly known
as polyploidy. When all the genomes present in a polyploid species
are identical, it is known as autopolyploid and the situation is termed
as autopolypioidy. In the case of allopolyploids, two or more
distinct genomes are present. Euploids may have 3 (triploid), 4
<i tetraploid ), 5 ( pentaploid ), 6 ( kexaploid ), 1 (heptapioid), 8 ( octaploid )
or more genomes making up their somatic chromosome number. In
case of autopolypioidy, they are known as autotriploid, auto-
tertaploid, autopentaploid, autohexaploid, autoheptaploid, auto-
-octaploid and so on, while in the case of allopolyploidy they
■are termed as allotriploid, allotetraploid, allopentaploid, allohexa-
ploid, alloheptaploid, allooctaploid etc. Amphidiploid is an allopoly-
ploid that has two copies of each genome present in it, and as a
consequence behaves as a diploid during meiosis. A segmental allo-
polyploid contains two or more genomes which are identical with
•each other, except for some minor differences
•HISTORY OF HETEROPLOIDY
The first variation in chromosome number (heteroploidy) dis-
coveted Wan experimental '•population was. the gigas mutant ia
Oenothera described by Lutz in 1907. The gigas was an autotetra-
ploid (4n). In 1910, Biakeste'e discovered the globe mutant of Datura
stramonium (dhatura) which was subsequently demonstrated by
Belling (in 1920) to be a trisomic ; this was the first reported case of
.aneuploidy. . The first autotetraploid was experimentally induced by
Winkler in 3916 in Solatium nigrum (black nightshade) ; he decapi-
tated the shoots, and' 'some- of the shodt buds arising from the calliis
thus' produced vverfe' tetrapldids. Wings' in 1917 suggested that
interspecific hybridization followed by chromosome doubling maif’&e
iihp'ortafit in the evolution' of new' specie's. The first' species symh$
sized experimentally was undoubtedly Triticale by Rimpau, in 1890;
irota a cross between Titricum and Secdle: But its chromosome
number was not studied till 1936. Therefore, the first synthetic species
about which the full information was furnished was Nicotians digi&ts
•obtained by spontaneous chromosome doubling bf the hybrid
from' the interspecific cross 'Nicotians' glutinosa («=12|) x
N. tabacum (»==24, <?). N. digluta was synthesized by Clausen and
■Oopdspeed in 1925. Chromosome doubling can be 'experimentally
obtained ih a relatively high frequency with colchicine treatment.
The chromosome doubling action of tolchicine was first described by
BJatesJee and Nebei in 1937 independently' of 'bach other, but thb
effect of cblchidne on mitosis was discovered in ’1955:
445
*
|
Polyploidy in Plant Breeding
ANEUPLOIDY
Of the various aneuploids, monosomies — in polyploid species,
such as, tobacco, wheat and oats — and trisomics— to diploid species,
e.g.. Datura , maize, bajra, tomato (L. esculetum ), rye (S. cereale),
pea (P. sat hum), spinach (S. oleracea) etc. —are most commonly used
iti genetic studies. Nuiiisomics are viable in a few highly polyploid
species only, e.g., wheat and oats; they are not viable even in tobacco
which is an allotetraploid. Therefore, we shall consider here trisomic
. and monosomic analyses. A trisomic is known as primary trisomic
if .the extra chromosome is the same as one of the haploid genome
that is,, it is not modified. In a secondary trisomic s the additional
chromosome is an isochromosome. In a?i isochromosome , the twe
arms of the chromosome are identical A tertiary trisomic has t
translocated chromosome as the extra chromosome. For the present,
we shall confine ourselves to primary trisomics.
Origin mi Produciioa
Spontaneous Aneuploids originate spontaneously at a low frequency,
.The earlier cases of aneuploidy were produced spontaneously in
experimental populations. Meiotic irregularities lead to the formation
of n + 1 and n~ i gametes, e.g , in Datura about 0.4 per cent of
pollen is likely to be n+1.
Triploid Plants The best sources of aneuploids are triploid plants..
Distribution of chromosomes at the first meiotic anaphase is
irregular leading to the production of a whole range of aneuploids in
the progeny.
Asynaptic and naptic Plants. In asynaptic and desynaptic plants,
few to all chromosomes are present as univalents at metaphase I of
meiosis. In the progeny of such plants, a relatively high frequency
of aneuploids occur.
. Translocation Heterozygotes. A3; 1 disjunction of the ring or the-
chain of four chromosomes in a translocation heterozygote would
produce one n±l and'one w — 1 gamete. As a result, in the progeny
of translocation heterozygotes a variable frequency of aneuploids-
are found.
Tetrasomic Plants. Tetrasomic (2«+2) plants would produce /i+i
gametes in considerable frequencies. Therefore, when they are
crossed with normal diploid or dlsomic (2n) plants, they produce a
high frequency of trisomics. Where possible, tetrasomics may be
maintained for the production of trisomics. ■ ■
Morphological and Cyfological Features . .■> . -
Aneuploids are generally weaker than diploids. Monosomies
do not survive in diploid species, and nuiiisomics do not survive even
in some polyploid species Ag., in tobacco. All the 21 nuiiisomics
Plant Breeding : Principles and Methods
4m
are available in tine wheat variety Chinese Spring. A foil series of
monosomies, that is, monosomies for each * chromosome of the
haploid complement, is available in wheat (Chinese Spring), tobacco
and oats. In diploid species, only trisomics survive ; full series of
trisomics are available in maize, barley, peas, tomato, bajra, spinach
and several other species. Trisomics vary considerably in vigour ;
some may be comparable to the normal diploid, e.g. 9 trisome 3 (that
is, trisomic for chromosome 5) in maize, while others are greatly
reduced in vigour. Aneuploids generally exhibit distinct morphology
and almost all the features of the plant are affected. Loss or gain of
different chromosomes usually produces distinct morphological
effects, but morphology is not always a reliable guide to the type of
■change at the chromosome level. Monosomies in wheat are often
comparable to normal plants in morphology. Therefore, all the
•plants suspected to be aneuploid must be analysed cytologically for
confirmation.
During meiosis, nuttisomics generally show regular bivalent
formation and chromosome distribution producing gametes with n— 1
chromosomes. Tetrasomics are relatively less regular, they usually
form one quadrivalent (involving four chromosomes) at metaphase I
which often separates 2 : 2 at anaphase L But sometimes the
separation of the four chromosomes of the. quadrivalent is not as
regular ; therefore, all the gametes produced by a tetrasomic are not
n+L In monosomies, one chromosome does not have a pair and
remains, as a univalent at metaphase L At anaphase I, the univalent
may move to one of the two poles, may lag and be lost, or may
divide (as in mitosis) into two chromatids which move to 'the
apposite poles. Due to irregular behaviour of the univalent, mono-
somies produce an excess of n~ 1 gametes, eg., wheat produces 75%
n — 1 gametes. In trisomics, the extra chromosome often forms a
trivalent with the other two homologous chromosomes, or may
remain as a univalent. The univalent behaves irregularly ; hence the
proportion of «-f.I gametes is usually less than 50 per cent.
Transmission of the aneuploid condition is very poor through
pollen grains, primarily due to a slower tube growth of aneuploid
pollen. As a result, normal pollen outcompetes the aneuploid one.
The transmission of aneuploidv through female gametes is consider-
ably higher. There is evidence that aneuploid seeds are generally
smaller, show somewhat reduced germination and' seedlings may
show lower viability ; all these lead to a reduced recovery of
aneuploid progeny.
Applications In Crop Improvement.
1. Aneuploids are useful in studies on the effects of loss or gain of
an entire chromosome or a chromosome arm on the pheno-
type of the individual. Their study has clearly demonstrated
that character expression is governed by a balance between a
large number of genes present in the genome, that is, loss or
gain of chromatin upsets the normal development.
Study of F 2 Generation . A strain carrying the dominant character is
of miliisomics, e.g, t the 21 nulli-
of such a cross would be all
The progeny
Aoeaploid Analysis for Locating Genes on Particular Chromosomes
Genes may be located on particular chromosomes by milli-
somic, moQOSomic or trisomic analysis. These analyses are briefly
described below.
Nillisoialc Analysis* Nullisomic analysis is restricted to a few
polyploid species like wheat. Four approaches may be used for
nulSisomic analysis : (1) absence of expression of dominant
character, (2) study of F* generation, (3) study of F 3 progenies,
and (4) chromosome substitution.
Absence of Expression of a Dominant Character . When a dominant
gene is located in a chromosome, the dominant character would not
be expressed, in the nullisomic for that chromosome. Genes for seed
colour and awn inhibition in wheat have been located in this manner.
Genes suppressing homoeologous pairing have been located on
•chromosome 5B because nu!li-5B plants show a., tendency for
homoeologous pairing and "multivalent formation instead of regular
bivalent- formation. But generally, genes present on homoeologous
chromosomes or on other chromosomes mask the absence of a domi-
nant gene in the nullisomic, le. s the nuilisomic may still show the
dominant phenotype. In such a case, study of the F 2 generation
would' be necessary.
Polyploidy in Phut Breeding
X
Aneuploids are useful in locating a linkage group and a gene
to a particular chromosome. By using a secondary or tertiary
trisomic, the gene may be located to one of the two arms of a
chromosome, or even to a part of the chromosome arm. The
most important application of aneuploids is in locating genes
on particular chromosomes ,* this will be considered in some
detail
3.
Study of aneuploids has shown the homoeoJogy between A, B
and D genomes of wheat ( T. aestivum\ since a chromosome of
A genome would compensate for the loss of the corresponding
chromosome from- the genome B or D. For example, tetrasomic
condition of 2B compensates for the nullisomic condition of
2A or 2D so that a tetra-2A nul!i«2B or 2D appears normal
4.
Aneuploids are useful in identifying the chromosomes involved
in translocation.
'5.
They are useful in the production of substitution lines. Chro-
mosome substitution may be desirable for studying the effects
of individual chromosomes of a variety or for the transfer of
the genes carried by specific chromosomes of a variety into
another one.
Plant Breeding : Principles and ‘M'ilhods-
44S
. , <r and the normal producing,
monosomies (nullisomics pr° aac Zygotes or union) ; the unpaired
« gametes giving rise to < in * n o«nal strain. These monosomic
chromosome is contributed oy a seoarate ly for each monosomic.
H Tullisomic for the critical chromosome-
plants are seifed and F» is ^^rnkjor' the critical chromosome-
F 2 family obtained from ^ stu dy) would show very
. (carrying the dominant gene un f L e J ninntq wit h t fa e recessive
■* g j**'f*s ' • ■.*' . „ ..ns!#* siuavt M'Utdiu otusrr j few plants
. (carrying the dominant gene unaer s ^ ^ rece$siye phenotype
with the recessive phenotype , - P u$e on selling monosomies
would all be J^dRullisomic plants (Fig. 24.1). The
produce normal, monosomic an . tfae cr j t ical chromosome
normal and monosomic pints ^ norma l parent ; only the
(carrying the dominant 8®. n ( : r have this chromosome (Fig. 24.2).
nullisomic progenies would not a 3 :T segregation
F. progenies from ”*“£omic^ producing a F, family with •
for the character. T h us the ° .(owing the recessive trait licks the
very small number of ptants sh | . phis observation should
chromosome carrying the gene un showing the recessive
be confirmed cytologically m thai me v
character should all be nullisomi
2 n
24%
(Normal)
2n-\
72%
(Monosomic)
2 «-»
• 1 %
(Monosomic)
2a—2
• 3%
(Nulfisomic)
- w aametes and progeny produced by monosomies
rig. 24.1. Frequency of different gamete ^ s J upon data collected so wheat
upon selfiog. The frequenci s d , _ n gamete ranges from' 0— SO
Pe? !e cent nS w“th S a mein of about 4 per cent, while about 75 per cent of
the female gametes 'areu-l*
„ „ _ . r„ rases, it may be desirable to study F s
Study of Fi Progenies In * Norm al disomic plants (plants having
progenies instead of F 2 P !ants - *1 Q e e 2n) from the E, genera-
ion a h r°e m sd?e g d U t e o rais^ proves. ^t^-itica, cress all such
atio will be obtained.
,, c„h<timtmn individual chromosomes from- a : given
SaSr; sjnxsz
s,r °'" M °°
Werent strain is known m chromosome
Polyploidy in Plant Breeding
NULLlSOMfC ANALYSIS
ff*AlR£NT$
CHROMOSOME
constitution
20 a
M0N0«0#M1C ANbA4»YWI
<j) MONOSOMIC
Chromosome substitution is easily done using nullisomics of '
the variety into which the chromosome is to be transferred (the re-
current parent). More commonly, monosomies are used in place
of nullisomics-, but the operation in this case is relatively more com-
plicated. The appropriate nullisomic is crossed with the strain (2«) ,
from which a given chromosome is to be transferred. The resulting I
monosomic Fi is backcrossed to the nullisomic (used as female),
yielding 96 per cent monosomic and 4 per cent nullisomic progeny.
The nullisomics are discarded. The monosomic progeny are repeat-
edly backcrossed to the nullisomic parent. After 6-7 backcrosses.
the monosomic plants are selfed ; they would produce about 24
normal disomic plants which would have one pair of chromosome:
from the normal parent (in the original cross). It may be pointed ou
that the Fj. and the monosomic plants in the subsequent generation
Locating a dominant gene on a particular chromosome by studying
the Fa generation from a cross with the nullisomic/monosomic
involving the chromosome carrying the gene in question. The results
in Fa from both monosomic and nullisomic analyses are the same.
The only difference lies in Fj ; nullisomics produce only monosomies,
while monosomies produce monosomies as well as normal plants
when crossed to a normal plant. Only monosomies are selfed to raise
the F2 ; they are identified by cytological analysis. The left hand pari
in the diagram depicts nullisomic analysis, while the right hand part
shows monosomic analysis. Fa generations from both the analyses
are identical. The figure depicts aneuploid analysis in wheat.
CHARACTER
RECESSIVE
CHARACTER
ALL MONOSOMIC
OOMiN'ANT CHARACTER
SELFED
1 _
20 0
75% MONOSOMIC
DOMINANT CHARACTER
SELFED
25*4 NORMAL
oomjnant
character
i
REJECTED
20 n 4-
A
24% NORMAL
dominant
CHARACTER
A
20 11 + —
MONOSOMIC
DOMINANT
CHARACTER
dominant
CHARACTER
RECESSIVE
CHARACTER
20 a
2% NULLISOMIC
RECESSIVE
character
450 Plant Breeding : Principles and Methoas
would receive an unpaired chromosome (for which the plants are
monosomic) from the normal parent.
Monosomic Analysis. Monosomic analysis is useful in locating genes
in particular chromosomes, and for chromosome substitution.
Monosomic analysis was first used in Nicotiana tabacum by Clausen
and was subsequently extended to other polyploid species.
Locating Genes in A Particular Chromosome. The complete set of
monsomics (used as female) is crossed to a normal disomic strain
carrying a dominant gene. In Fi, 25% of the plants would be 2n ;
they are rejected. The remaining (75%) plants would be monosomies
(identified by c> tological analysis), which are selfed to produce the
Fs generation. In the F 2 derived from the monosomic for the critical
chromosome (carrying the gene under study), all the plants would
show the dominant character ; those showing the recessive trait
would be nullisomics (Fig. 24.3). In tobacco, nullisomics do not
survive, hence plants showing the recessive trait are absent in the
critical family.
Chromosome Substitution. The appropriate monosomic is used as
the recurrent parent in a backcross programme with the strain from
which the chromosome is to be tranferred (the donor parent). The
monosomic (used as female) is crossed to the donor parent. The Fj
has about 75% monosomic progeny, which are monosomic for the
chromosome to be transferred. The monosomic Fj plants are utilized
in one of three ways.
1. Monosomic Fi plants are selfed to produce In plants which have
the critical chromosome in disomic condition, i.e., having two
homologues. Disomic plants are backcrossed to the appro-
priate monosomic of the recurrent parent. Monosomic progeny
from the backcrosses are handled in the same manner as the
monosomic F x progeny. Thus each backcross generation is
followed by a selfed generation, and thus 6 backcrosses would
require 12 generations. At the end of backcrossing, monosomic
progeny are selfed and 2 n progeny are isolated ; these would
be disomic for the critical chromosome, and w ill constitute
the substitution line.
2. The monosomic line of the recurrent parent may be monosomic
for a telocentric chromosome or an isochromosome, which can
be recognised from the normal chromosome under the micro-
scope. If such a line is used as the recurrent parent, there is
no need for selfing ; backcrosses follow each other in succes-
sion, and 6 backcrosses require only 6 generations. The mono-
somic recurrent parent is used as female, while those from Fi
and the backcross progeny are used as male.
3. Even when normal monosomic lines arc used as k urrent
parent, backcrosses may be made in succession without selfing
the monosomic progeny (as in the case of item 2). One out of
eve<-- 75 monosomies in such a case would be monosomic for
the chromosome from the recurrent parent ; this chance in 6
backcrosses would be 1 out of 12.
Polyploidy in Plant Breeding
null; som ic
'( RECURRENT PARENT }
Q NORMAL <OI‘JOM(C,
in#n«*e current parent*.
■CHROMOSOME
--CONSTITUTION
Fi backcrossed to the
nullisomic recurrent parent,
20,2
NUUIBOMiC recurrent
PARENT
) 20 0 T -
MONOSCft.tiC
20 Q
NULLISOMIC
f REJECTED )
,X 20 a y
/ NULLlSCMtC
( (RECURRENT
' PARENT)
Monosomic plants selec-
ted and backcrossed to
the recurrent nullisomic
parent.
TO.K
<NULUSOM!C,
rejected;
20 E 4 — -
{MONOSOMIC, aCLFCD)
20 s +
{NORMAL OiSOMiC ;
THE SUBSTITUTION
linejchromcsome
CARRYING GENS A
TRANS? ERRED IN THE
OENSTIC back ground
OF THE RECURRENT
PARENT)
20 3
NUIUSOMIC
(REJECTED)
Fig. 24.3. Chromosome substitution using a nullisomic for the critical chromo-
some as the recurrent parent. If a monosomic recurrent parent is
used, monosomies from Fi and each backcross generation are selfed
to isolate normal disomic plants these are then backcrossed to the
monosomic recurrent parent.
Trisomic Analysis. Trisomic analysis is applicable to diploid species
where monosomic. or nullisomic analyses are not possible. The
strain carrying the gene to be located on a particular chromosome
is crossed (as a male) to each of a complete set of trisomics (used as
female). The Fi would have trisomic and disomic plants ; disomies
are discarded. Trisomic Fj plants are' either selfed or testcrossed to
a recessive disomic strain. The resulting F a or the testcross progeny
would show a 3 : 1 or 1 : 1 segregation, respectively, for the charac-
ter if it is not governed by a gene present on the chromosome which
is in the trisomic state. But the ratio would be significantly different
from 3:1 or 1 : 1 if it is present on the trisomic chromosome.
|| m
452 plant Breeding : Principles and Methods
Trisomic analysis has been done in maize, barley, tomato and some-
other crops.
Limitations of Aneuploid Analyses
1 Tt is necessary to produce and maintain a complete set of
aneuploids. Production, identification and maintenance of
aneuploids requires elaborate cytogenetic analysis, which is
difficult, time consuming and requires considerable skill.
0 Maintenance of aneuploids is complicated by the phenomenon
2 - ‘W> dm ° ,cs ,hat s °?°/ r ,he
orogeny of an aneuploid plant would become meop o.d for a
different chromosome as compared to the parent plan . Univa*
lent shift generally occurs in monosomic lines and is a result ot
univalent formation in a chromosome other than a for
which they are monosomic. Therefore, eytological analysis end
Si'S,, an integral part of the aneuploid pro-
grammes. .
1 During aneuploid analysis and chromosome substitution,
3 ' cvtololical analysis must be carried out for accuracy. This
Slvfs a considerable eytological work and makes aneuploid
analysis a time consuming and tedious task.
AUTOPOLYPLOIDY
In autopolyploidy are included monoploidy, t r t p 1 o idy , tet r a-
ploid, and higher pToTdytv'e.s ll d
shall°be considered separately. Other cases of autopolyploidy are
‘3uc b ed directly or
is briefly considered .below. » * do not constitute polyploidy.
Bu^fvliw of their specialized application in crop improvement,
they are included here for discussion.
Origin And Production of Doubled Chromosome Numbers
Cells/individuals having doubled chromosome numbers may
• • , re in one of the following several ways : (0 spontaneous,
Q due to treatment with physical agents, (3) regeneration m vitro,
(4) cblchTcine treatment, and (5) other chemical agents.
<$xnntaneons Chromosome doubling occurs occasionally m somatic
Ssuefand unreduced gametes are also produced m low frequencies.
Suction of unreduced gametes is promoted by certain genes, e.g. t
genes causing complete asynapsis or desynapsis.
Production of Adventitious Buds. Decapitation in some p.ants leads
to callus development at the cut end of stem. Such a callu s ha
come noivoloid cells, and some of the shoot buds regenerated from
th?c5?us P may be polyploid. This is of common occurrence in
Slnaceae where 6-36 per cent of adventitious buds are reported
to be teSploid. The frequency of polyploid buds may be increased
453
polyploidy in Plant Breeding
'by the application of 1 per cent IAA at the cut end as it favours
callus development.
Physical Agents. Heat or cold treatments, centrifugation and X-ray
or ” gamma-ray Irradiation may produce polyploids in low fre-
quencies. Tetraploid branches were' produced in Datura in response
to cold treatment. Exposure of maize (Z. mays) plants or ears to a
temperature of 38-45°C at the time of the first division of zygote
produces 2-5 per cent tetraploid progeny. Heat treatment has’ been
successfully used in barley (H. vulgar e ), wheat (7. aestivum), rye
(S. cereale) and some other crop species.
Regeneration In Vitro. Polyploidy is a common feature of the cells
during in vitro culture. Some of the plants regenerated from callus
and suspension cultures may be polyploids. Plants of various ploidy
have been regenerated from callus cultures of Nico liana, Datura , rice
(O. sativd) and several other species.
Colchicine Treatment. Colchicine treatment is the most effective and
the most widely used treatment for chromosome doubling. It has been
used with great success in a large number of crop species belonging
to both dicot and monocot groups. Colchicine is a poisonous chemi-
cal isolated from seeds (0.2-0. 8%) and bulbs (0.1-0. 5%) of autumn
crocus (Colchicum autummle). It is readily soluble in alcohol, chloro-
form or cold water, but is relatively less soluble in hot water. Pure
colchicine is Caa^eOgN, It blocks spindle formation and thus inhibits
the movement of sister chromatids to the opposite poles. The result-
ing restitution nucleus includes all the chromatids ; as a result, the
chromosome number of the cell is doubled. Since colchicine affects
only dividing cells, it should be applied when the tissues are actively
dividing. At any given time, only a small proportion of cells would
be in division, hence repeated treatments should be given at .brief
intervals to double the chromosome number in a large number of
cells of the shoot apex. The polyploid and diploid cels present in
a shoot-tip • compete with each other and diploid cells may often
outcompete the polyploid ones. The degree of competition varies
from species to species and even among varieties within species.
The method of colchicine application varies considerably. The'
different methods of treatment are briefly summarised below. Colchi-
cine- is usually applied as an aqueous solution. Colchicine is rela-
tively unstable in an aqueous solution. Therefore, it is important to
use freshly prepared aqueous solutions of colchicine.
1. Seed Treatment may be used for 1 to 10 days with concentra-
tions from 0.001 to 1 per cent ; 0.2% is more common. Seeds
are generally soaked in a shallow container to facilitate aera-
tion.
2. Seedlings may be treated in a young stage. Germinating seeds.,
may be inverted so that only young shoots' are exposed to
colchicine and roots are protected. Treatment duration " may
range from ' 3-24 hours.
450
454 Plant Breeding :• Principles and Methods
3. Growing shoot apices are commonly treated with 0.1 to 1 per
cent colchicine, which is applied by brush or with a dropper.
The treatment is repeated once or twice daily for a few days.
A small cotton wool piece may be placed at the shoot-tip
which is daily soaked with colchicine solution. Alternatively,
0.5 to 1 per cent colchicine mixed in lanoline paste may be
smeared on the shoot apex ; this treatment is repeated 2-3
times per week.
4. In woody plants * 1 % colchicine is generally used for applica-
tion on shoot buds. A small quantity of a wetting agent is
added in the colchicine solution for a better wetting and pene-
tration.
5. Special methods for the treatment of cereals and gr asses have
been developed. These methods aim at bringing the colchicine
solution in contact with the shoot apical meristem.
Other Chemical Agents. Several other chemicals have polyploidizing
effect Notable among them are, acenaphtbene, 8-bydroxyquinoline
and nitrous oxide. These chemicals are much less effective than
colchicine and are not commonly used.
Colchicine treatment may produce a low frequency of variable
chromosome numbers (other than the doubled number) including
haploidy, e.g, $ in jowar (S. bicolor), tomato (Z,. esculentum) etc.
Colchicine may also induce mutations without influencing the chro-
mosome number. Mutants induced by colchicine generally breed
true. These responses to colchicine may be considerably affected by
the genotype of the variety being treated, light conditions, mineral
nutrition etc. Available evidence suggests that colchicine may induce
mutation alone, or mutation followed by chromosome reduction
coupled with chromosome doubling ; the latter produces true-breed-
ing mutations.
Morphological Aid Cytologies! Features of Autopolyplofds
Morphological features of polyploids vary to some extent
from one species to the other. Some general features are summarised
below.
L Polyploids have larger cell size than diploids. Guard cells of
stomata are larger, and the number of stomata per unit area is
lower in polyploids than in diploids.
2. Pollen grains of polyploids are generally larger than those of
the corresponding diploids.
3. Polyploids are generally slower in growth and later in
flowering.
4. Polyploids usually have larger and thicker leaves, and larger
Sowers and fruits which are usually less in number than in- the
diploids .
5. Polyploids generally show reduced fertility due to irregularities.
won]
mon
Mon
in p
Mon
and "
Loca
mom
carry
they .
(Men
Ft ge
chroi
show
woul
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Chm
the r<
whicl
monc
has a
chroi
in on
Polyploidy in Plant Breeding 455
daring meiosis and due to genotypic imbalance leading to
physiological disturbances.
6 . In many cases, autopolyploidy leads to an increase in general
vigour and vegetative growth. But in some cases, polyploids
are smaller and weaker.
7. Different species have different levels of optimum ploidy. For
sugarbeet (Beta vulgaris ), the [optimum level is 3x, while for
timothy grass ( Phleum pratense) it is between 8-1 Ox
8 . Autopolyploids generally have a lower dry matter content than
diploids. As a result, the increased size of many polyploids
may not represent an increased dry matter production, e.g
tetraploid turnip (. Brassica rapa) and cabbage (B. oleracea ).
The c>tological features vary with the level of ploidy. In mono-
plaids , the chromosomes do not pair and their distribution at
anaphase I is random leading to an almost complete sterility. Some
functional gametes with n chromosomes may be produced which
may give rise to 2 n progeny. In triploids , a variable number of
trivalents, bivalents and univalents are produced. At anaphase I,
chromosome separation in tri talents and distribution of uni-
valents is irregular producing a range of aneuploid gametes. As
a result, they produce various types of aneuploid progeny, e.g. f
trisomics, double trisomics etc. Autotriploids are generally highly
sterile, e.g., watermelons, banana (Musa par adisiaca) etc., but in.
some cases they are highly fertile, e.g., spinach (. Spinacea oleracea ).
Auiotetr aphids show quadrivalents, trivalents, bivalents and univa-*
lents at raetaphase I. They show variable fertility in different species,
but the fertility is generally lower than that in diploids. In the pro»
geny of tetraploids, the majority of plants are tetraploids, but some
plants may show variation in chromosome number. There is consi-
derable evidence that fertility of autotetraploids can be improved
through selection. This has been achieved in many cases, e.g., maize
(Z. mays), tori a (B. campestirs var. tor id), rye (S. cereale), bajra
{. P . americanum ), rice (O. saiiva) etc. In the case of tetraploid Petkus
rye. the fertility increased from 60% to 75% after 6 years of
selection.
Segregation m Autotetraploids
Segregation in autotetraploids is much more complex than in
diploids. This topic has been considered by Allard (1960) in some
detail, and in this text a brief summary will be given. In an auto-
tetraploid, 4 chromosomes are homologous to eachfother, hence each
gene has 4 copies. A simplex individual has one dominant and 3
recessive alleles ( Aaaa ) 8 a duplex has 2 dominant and 2 recessive
alleles (AAaa), a triplex has, 3 dominant and 1 recessive allele
(. AAAa ), a quadruplex has - all dominant alleles (AAAA), while a
nt lliplex has none (aaaa). \ .
- Let ■ us consider segregation in a simplex (Aaaa), When: the
gene A is located close to centromere, crossing over between the
woui
mom
Mom
in p
Mob
and i
Local
mom
carry,
they i
(idee'
Ft ge
chroi
show
wouh
sxirvr
critici
Chrot
the r«
The two assumptions in the above estimation are related to the
regularity of quadrivalent formation and of complete or no linkage
between centromere and the gene in question. These assumptions are
rarely, if ever, fulfilled. Generally, quadrivalents are not regularly
produced , a variable number of bivalents, univalents and trivalents
are also formed. Further, genes and centromeres are generally
partially linked. Therefore, the actual gametic and zygotic frequen-
cies are intermediate between the two extremes set by random
chromosome and random chromatid segregations, since one, the
other or both of the assumptions are bound to be incompletely
AAAA
i
AAAa
24
AAaa
174
Aaaa
360
aaaa
225
784
plant Breeding : Principles, and Methods
centromere and the gene would not take place. In this case, both
the sister chromatids of each chromosome are attached to the same
centromere and would move to the same pole at anaphase I. At
anaphase II, the sister chromatids carrying the dominant allele would
move to the opposite poles. Therefore, the two dominant alleles do
not reach the same gamete, i.e., AA gamete is not produced^ Such a
segregation is known as random chromosome segregation. Complete
chromosome segregation (complete absence of crossing over between
centromere and the gene in question) in a simplex would produce
two types of gametes a and aa, in the ratio 1:1. Self-pollination
of a simplex would produce three genotypes AAaa, Aaaa , and aaaa,
in the ratio 1:2:1, giving the phenotypic ratio of 3 : 1 if a single
dominant allele A is able to produce the dominant phenotype. It is
assumed that at metaphase I only quadrivalents are formed and that
separation of chromosomes is random.
When crossing over occurs between the centromere and the
gene, the two sister chromatids carrying the dominant allele become
attached to two different chromosomes (or centromeres). The two
chromosomes involved in the crossing over (each now carrying a
single dominant allele) may move to the same pole at anaphase I.
At anaphase II, the two chromatids carrying the A allele may move
to the same pole, and thus end up in the same gamete producing an
A A gamete. This type of segregation is termed as random chromatid
segregation. Random chromatid segregation produces 3 types of
gametes, AA, Aa and aa, in the ratio 1:12:15. It is assumed that
all the chromosomes are associated as quadrivalents, and that 50
per cent crossing over occurs between the gene and the centromere.
Selfin g of a simplex in such a case is expected to produce the follow-
ing progeny.
Quadrupiex
Triplex
Duplex
Simplex
Nulliplex
457
Polyploidy in Plant Breeding
Sote of Aatepolyflolij I® EfoMim
Autopolyploidy has contributed to a limited extent in evolution
of plant species. Some of our present day crop species are autopoly-
ploids, e.g.t potato (4x), peanut (4x), coffee (4x), alfalfa (4x\ banana
(3x) and sweetpotato (6x). Autotetrapioids appear to have been
more successful as crops than other forms of autopolyploidy
(Table 24.2). In addition, many forage grasses and several ornamen-
tals are most likely autopolyploids.
Table 24.2. Autopolyploid crop species.
Common name
Scientific name
Somatic chromo-
some number (2 n)
of the cultivated
form
Somatic chromosome
number of related
wild species
Potato
Solanum tuberosum
48 (4x)
24 (2x) form of
S. tuberosum
Coffee
Coffea arabica
44 (4x)
22, 66, 68
Alfalfa
A4edlcago sativa
32 (4x)
14, 16, 32
Peanut
Arachis hypogaea
40 (4x)
Banana
Musa sapient urn
(M, paradisiaca)
33 (3x)
22
Sweetpotato
Ipomoea batatas
90 I6x)
Application of Autopolyploidy m Crop Improvement
Autopolyploidy has found some valuable applications in crop
improvement. These are briefly summarised below.
M'oaoploids and Haploids. Monoploids are weaker than diploids
and are of little agricultural value directly. But they are of great
interest because they offer certain unique opportunities in crop
improvement. (!) They are used for developing homozygous diploid
lines, following chromosome doubling in two years. This greatly
reduces the time and labour required for the isolation of inbreds and
purelines. (2) They may be useful in the isolation of mutants because
the mutant allele (even if it is recessive) expresses itself in Mi due to
a single dose of the gene in somatic tissues. Chromosome number of
mutants may be doubled to produce homozygous mutant lines in a
•single generation. (3) Since desirable gametes are more frequent (p)
than desirable zygotes (/?*), selection based on haploids or haploid-
derived diploids may be expected to be more efficient than that
based on diploid (zvgote-derived) plants.- There is some evidence
that this may be so. And (4) in autotetrapioids like potato, breeding
is relatively much easier at the haploid (2x) level than at the tetra-
ploid level (4x). For comparison, consider segregation in an
autotetraploid and in a diploid. There is an increasing tendency to
breed potato varieties at th~ haploid level and then double their
chromosome number to obtain tetraploid varieties.
Monoploids and haploids occur spontaneously (e.g., in maize.
Chapter 19) in low frequencies, may be induced from pollen grains
through callus formation or embryoid production (Chapter 26) and
450
.
woul
mom
Morn
in p
Moo
and ^
Loca<
mans
carry
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Chrot
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wfaicl
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Plant Breeding f Principles and Methods
bv chromosome elimination in certain interspecific crosses, e.g.,
Hordeum bulbosum X H. vulgate (Chapter 25). In the first method,
the recovery of haploids is generally very low (1 m 1,000 plants or
lower) ; But the latter two methods produce a relatively high frequ-
encv of haploids in case of those species for which appropriate
techniques are available. It may be pointed out that the latter two
methods are not applicable to many crop species as yet.
Triploids Triploids are produced by hybridization between tetraploid
and diploid strains. They are generally highly sterile, except in a few
cases This feature is useful in the production of seedless water-
melons. In certain species, they may be more vigorous than the
normal diploids, e.g.. in sugarbeets. These two examples are described
in some detail.
Seedless watermelons are grown commercially in Japan. They
are produced by crossing tetraploid (4;*% used as female) and diploic!
(2x used as male) lines, since the reciprocal cross (2xX4x) is not
successful. The triploid plants do not produce true seeds ; almost
all the seeds are small, white rudimentaiy structures like cucumber
(Cucumis sativus ) seeds. But a few normal sized seeds may occur,
which are generally empty. For good fruit setting, pollination is
essential For this purpose, diploid lines are planted m the ratio
1 diploid : 5 triploid plants. There are several problems, viz., genetic
instability of 4x lines, irregular fruit shape, a tendency towards
hollowness of fruits, production of empty seeds and the labour
involved is triploid seed production (by hand-pollination).
Triploid sugarbeets {B. vulgaris) produce larger roots and
more sugar per unit area than diploids, . while tetraploids produce
smaller roots and lower yields than diploids. Apparently, 3x is the
optimum level of ploidy in sugarbeets Triploid sugarbeet varieties
have been grown commercially in Europe and Japan, but their
popularity is declining rapidly. The triploid varieties are mixtures
of triploid, diploid and other ploidy level plants. Seed product-
ion of triploid sugarbeet is difficult because the beet Sower is small.
Triploid seed may be produced in one of two ways.: (1) using 4x
plants as female and 2x as male or (2) using 4x as male and 2x as
female. The first combination gives lower seed yield but a higher
proportion of triploids, while the second gives a higher seed yield
but a lower proportion of triploids. Commercial triploid sugar-
beet seed is produced bv interplanting 4x and 2x lines in the ratio of
3-1 and seed from both 4x and 2x plants is harvested. This
seed consists of about 75% triploid (3x) seeds. Triploid sugarbeet
may give 10-15 per cent higher yields than diploids.
Tetraploids. Autotetraploids have been produced in a large number
of crop species and have been extensively studied in several cases.
Tetraploids may be useful in one of the following ways. : useful in
breeding, improved quality, overcoming self-incompatibility, making
distant crosses and used directly as varieties.
In banana (M. sapientum), autotetraploids are inferior to
triploids in that they have weaker leaves and increased fertility. But
459 *
Polyploidy in Plant Breeding
they offer the only available chance of adding disease resistance to
commercially successful varieties. In banana, autoteraploids are
produced by chance fertilization of m unreduced tripioid egg (AAA)
by a haploid pollen from a disease resistant diploid parent. A large
number of such tetraploids have been produced, but they have not
yet gained any commercial success. This is an unusual case where the
autotetraploidy is the only practical approach to breeding an other-
wise successful crop species.
Some autotetraploids may be superior in some quality characters
to their respective diploids, e.g., tetraploid maize has 43% more
carotenoid pigment and vitamin A activity than the diploid. Some
teraploids may be more hardy than diploids. However, it is-
impossible to predict the performance of tetraploids, and a superior
diploid may not necessarily produce a superior tetraploid. The
tetraploids may be superior, inferior or comparable to the correspon-
ding diploids in quality and hardiness ; the actual response has to-
be determined experimentally.
Autotetraploidy is able to overcome self-incompatibility in
certain cases, e.g. t some genotypes of tobacco (Nicotiana sp.) and
white clover ( Trifolium repens), Petunia etc. Certain distant crosses
are npt successful at the diploid level, but are relatively successful at
the autotetraploid level, e.g. 9 4x Brassica oleracea X B. chinensis is
successful, but when B. oleracea is diploid it is unsuccessful.
Similarly, autotetraploids of certain Solarium species produce hybrids-
with S. tuberosum while diploids do not.
Autotetraploids are larger in size and are more vigorous than '
diploids. Antotetraploid varieties of forage crops have been consi-
derably successful. The most successful examples are, tetraploid red
clover (Trifolium pratense) and ryegrass (Lolium per erne ). Other'
examples are : tetraploids of alsike clover (Trifoiium hybridum ,
variety Tetra) and berseem (Trifoiium alexandrium , variety Fuse
Giant Berseem). Autotetraploid red clover and ryegrass are more
vigorous, digestible and palatable, and have greater resistance to*
nematodes as compared to the diploids. Autotetraploid turnips
(3. rapa) and cabbages (B. oleracea) are larger in size, but they .also
have more water content than the diploids ; thus they are' not
commercially attractive. Many ornamentals are autotetraploids. la
cases of ornamentals, increased flower size, and longer Sewering:
duration of the tetraploids are desirable. Posa Giant Berseern is-
the first polyploid variety released for general cultivation in India.
It yields 20-30 per cent more green fodder than the diploid berseem-
varieties.
In case of crops where seed is the commercial product, # auto-
tetraploidy has been much less successful. The chief difficulty is the
high sterility and genetic instability of autotetraploids. Fertility cm
be improved through breeding and selection, but the progress is slow*
Autotetraploids have been explored in several crop species but the
most successful case is that of rye (S. cereale) where tetraploid varie-
ties have been released for cultivation (e.g, t Double Steel, Tetra Pet.-'
lens). Other extensive programmes on autotetraploidy are on crops
Plant Breeding ■ ; Principles and Methods
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like barley (H. vulgare) and jowar (S, bicolor) where larger grains,
■increased protein content and higher yields are the objective. After
many years of extensive breeding, some success in achieving these
;goaJs has been realised* Some generalisations may be made about
autopolyploidy.
h Autopolyploidy is more likely to succeed in species with lower
chromosome numbers than in those with higher chromosome
numbers. .
2. Cross-pollinating species are generally more responsive than
self-pollinating species.
3. Crops grown for vegetative parts are more likely to succeed as
polyploids than those grown for seeds.
.Limitations of Aiatopoljploldy
I. The larger size of autopolyploids is generally accompanied with
a higher water content. As a result, autopolyploids of the crop
species grown for vegetative parts do not always produce more
dry matter than the respective diploids. For example, tetraploid
turnip (B, rapa) and cabbage (B. oleracea) outyield the diploids
in fresh weight, but are comparable, or even inferior, to them’
in terms of dry matter production. .
.2. In crop species grown for seed, autopolyploids show high steri-
lity accompanied with poor seed set. Consequently, the larger
seed size of autotetraploids does not generally lead to an
increased seed yield per unit area.
3. Fertility in autotetraploids can be increased by hybridization
and selection at the tetraploid level. But due tc the complex
segregation in autotetraploids, progress under selection is slow.
It would take many years to raise the fertility to acceptable
levels.
4. Monopioids and triploids cannot be maintained, except
through clonal propagation. The progeny of triploids and
tetraploids are variable in chromosome number since they pro-
duce aneupiotd gametes as well. Triploids have to be' regu-
larly produced by crossing 4n X 2 n plants. Maintenance of
tefraploids is somewhat less difficult. Thus genetic insta-
bility of autotriploids and autotetraploids makes their main-
tenance difficult, and commercial seed production presents
many problems.
:5. The hope that polyploidy would help to create new agricul-
tural types at will was entirely misplaced. New polyploids (raw
polyploids) are always characterised by a few or more un-
desirable features, e.g , poor strength of stem in grapes (Vitis
vinifera ), irregular fruit size in watermelons etc. Thus new
polyploids can rarely be used directly in crop production. A
considerable improvement through hybridization and selection
essentia! to remove these defects.
461
polyploidy in Plant Breeding
6. Effects of autopolyploidy cannot be predicted. Therefore, one
has to try and find out if it would be of some use in a given
crop species;
ALLOPOLYPLOIDY
Allopolyploids have genomes from two or more species. Seve-
ral of our crop plants are allopolyploids. Production of allopoly-
ploids has attracted considerable attention ; the aim almost always
was the creation of new species. Some success has been obtained as
is evident from the emergence of Triticale as a new crop species in
some areas, and the promise shown by some other allopolyploids,
e.g., Raphanobrassica and some allopolyploids of forage grasses.
Origin And Production of Allopolyploids
The present-day allopolyploids were most likely produced • by
chromosome doubling in Fi hybrids between two distinct species
(distant hybrids) belonging to the same genus or to different genera.
Chromosome doubling might have occurred in somatic tissues
due to an irregular mitotic cell division leading to the formation
of allopolyploid sectors either in the apical meristem or in the
axillary buds ; the latter would produce allopolyploid branches.
Sexual progeny from such branches would be allopolyploids. Alter-
natively, irregular meiosis may lead to the production of unreduced
gametes which may unite to produce allopolyploid progeny. A
complete- failure of pairing may be followed by the inclusion of all
the chromosomes in a single nucleus at telophase I. This would lead
to the production of two unreduced gametes after telophase II.
. In experimentally produced distant hybrids, both the processes
generating allopolyploidy have been observed. In many distant
hybrids that were highly sterile, occasional branches were highly
fertile and had twice as may chromosomes as those present in the Fi
hybrid, e.g., Fi from the cross Primula verticillata X P. floribunda
produced a fertile branch with twice the chromosome number. The
progeny obtained from this fertile branch were allotetraploid and
were named as P. kewensis. In many other distant hybrids, occasio-
nal unreduced gametes are produced which unite to give rise to
allopolyploid pirogeny. The allotetraploid Raphanobrassica was ob-
tained in this manner ; the Fi from the cross B. okracea (cabbage) x
Raphanus sativus (radish) was almost completely sterile, but
produced a few seeds that gave rise to the allotetraploid Rophano-
brassica.
Experimental production of allopolyploids is achieved by doubl-
ing the chromosome number of distant hybrids with the help of
colchicine or some other agent. The allopolyploids produced by
man are often termed as synthetic allopolyploids because they are
produced (or so to say synthesized) by man from two distinct species
of plants. Thus the production of an allopolyploid involves two
steps : (1) production of Fi distant hybrid, and (2) chromosome
TRlTlCUM TURGIOUM X SEGALS CEREALE
AA 8®
COLCHICINE
(CHROMOSOME
DOUBLING}
The Fi hybrid would be
highly sterile; chromosome
number of the hybrid is
doubled by using colchicine
.AMPHiDlPLOiO
AA SS RR
TRfTICALE HEXAPLOIDE
(ALLOHEXAPLOlDj
Fertile allopolyploid
(amphidiploid) species distinct
from the two parental
species
Fig. 24.4. Experimental production (synthesis) of an allopolyploid. The allo-
polyploid synthesised here is an allohexanloid, the hexapolyploid
Triticak ( Triticak haxaploide )•
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4g2 Plant Breeding | Principles and Methods
doubling (Fig. 24,4). Many distant crosses not possible .previously
have been successfully made with the help of in vitro techniques. ^
Chromosome doubling does not appear to be a serious problem in -y
most of the cases since most of the plant species respond to colchi- j
cine. Some species may respond poorly to colchicine ; in such cases
other agents available for chromosome doubling may be useful.
Morphological And Cytological Features of Allopolyploids
Allopolyploids generally combine the morphological and physio-
logical characteristics of the parent species. But it is very difficult to
predict the precise combination of characters that would appear in
the allopolyploid species. For example, the aim in producing
Raphanobrassica was to synthesize a crop species that would com-
bine the root of radish (R. sativus) and the leaves of cabbage
(B. oleraced). Raphanobrassica did combine the characteristic root
and shoot systems of the two parental species, but in the opposite
direction, that is, it had leaves like radish and roots like cabbage !
On the other hand, Triticale has combined the favourable features
of the two parental species, i.e., the hardiness of rye (S. cereale)
and the yielding ability of wheat (Triticum sp.). In general, allopoly-
Two distinct species ar®
hybridized
PARENTAL
SPECIES
GENOMES
GAMETES
463
Polyploidy in Plant Breeding
ploids are more vigorous than diploids, but this also is not true in
all the cases. The natural distribution of allopolyploids is usually
different from those of the ancestral diploid species. This would
indicate that allopolyploids differ in their adaptability from their
parental species. Often they are hardier than the parental species,
but rarely they are able to displace the parental species from the
areas of their adaptation. Thus evolution of allopolyploidy in nature
has been favoured by the availability of new ecological areas for
the establishment of allopolyploids.
Another feature of allopolyploids is that many of them are
•apomictic. Apomictic species are common in grasses, e.g. 9 Poa .
Apomicts are known in many other groups of plants, e.g. f Taraxa ■
cum 9 Parthenium , Rubus, Fritiilaria , Tulipa , Solarium , etc. Apomixii
arises in response to the near complete sterility of interspecific
hybrids. Such sterile plants are able to reproduce asexually through
apomixis.
The characteristics of allopolyploids would result from an
-interaction between the genetic systems of the two parental species
that have evolved independently of each other. The nature of the
interaction between the two systems may vary greatly from one
allopolyploid to the other, which probably is partly responsible for
the difficulty in predicting the effects of allopolyploidy. Another
factor that is involved in allopolyploids- Is that two genomes of a
species are transferred into the cytoplasm of another species. The
cytoplasm-nucleus interactions may be of some importance in deter-
mining the characteristics of allopolyploids.
Chromosomes of distinct species may be expected to have
differentiated to various degrees ; chromosomes of closely related
species would be more similar to each other than those of -unrelated
species. Thus chromosomes of related species would be partially
similar to each other (as against exactly identical homologous chro-
mosomes) and are known as homoeologous chromosomes . Successful
hybridization between two species shows some degree of relationship
between- them ; hence interspecific hybrids would have two or more
homoeologous (not homologous) chromosomes. These homoeo-
logous chromosomes may show various degrees of chromosome
■pairing depending upon the degree of homoeology. After chromo-
some doubling, the allopolyploid would have two homologous
chromosome for each chromosome present in the Fj hybrid, which
is the same situation as in the diploid species. Such an allo-
polyploid, therefore, is commonly known as amphidiploid or
amphiploid .
When the two genomes in an amphidiploid are divergent, that
is, they have little homoeology, chromosome pairing would be highly
regular and only bivalents would be produced at metaphase I.
Such amphidiploids would be highly fertile, would be stable both
genetically and cytologieaUy and would show only bivalents at
metaphase I. But if the two genomes are sufficiently homoeologous
to show some chromosome pairing in the F x hybrid, the amphidi-
ploid would show a variable number of quadrivalents. Such
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4 g 4 Plant Breeding : Principles and Methods
amphidipioids would be partially ferrile, unstable genetically and
cytoiogically* and would show some quadrivalents at metaphase I,
However, these generalisations are not always true, and many
exceptions are known. Sterility is a common feature of arnphidi-
ploids and is most likely produced by physiological disturbances
resulting from genetic imbalance. Irregular chromosome pairing, if
present, is also a contributing factor.
Fertility of allopolyploids can be improved through hybridi-
zation and selection. Selection not only improves chromosome
behaviour, but it also improves the gene combinations through
recombination. Thus there is an increase in bivalent formation as a
result of selection so that established polyploids become identical to
diploids in terms of chromosome beaviour, i.e., they become
diploidized. The process due to which allopolyploids shows diploid-
like behaviour, i.e., only bivalent formation, is known as diploidi-
zation. Recombination leads to the development of gene combina-
tions that improve physiological and cytological functions in
allopoivploids. Natural allopolyploids have evolved highly efficient
genetic'svstems that ensure regular bivalent formation by preventing
pairing between homoeologous chromosomes. An excellent example
IS provided by the homoeologous pairing suppressing action of
chromosome 5B in wheat. Wheat (T. aestivum) lines rncking-
chromosome 5B, e.g., nulli-5B, show homoeologous pairing and
multivalent formation. Similar genetic systems that suppress
homoeologous pairing occur in the other established allopolyploids
like oats (A. sativa), tobacco (N. tabacum ) etc.
Role of Allopolyploidy in Evolution
Allopolyploids have been more successful as crop species than
autopolyploids. Many of our present-day crop species are allopoly-
ploids (Table 24.3). Allopolyploidy has contributed to a great
extent in the evolution of plants. This is evident from the wide-
spread occurrence of allopolyploidy in various genera of plants and
from the great success allopolyploids have enjoyed in natural
populations. It is estimated that about one-third of the Angiosperms
are polvploids, and by far the vast majority of them are allopoly-
ploids. 'in 1917, Winge postulated that allopolyploidy was
important in evolution ; subsequent studies have clearly shown the
validity of his suggestion.
It has been possible to trace back the evolutionary history of
many allopolyploid crop species, and the diploid parental species
have been identified with some degree of certainty. The identifica-
tion of parental diploid species is primarily based on pairing bet-
ween the chromosomes of the diploid and the allopolyploid species.
When the chromosomes of a diploid species pair with some of those
of the allopolyploid species, homology between the chromosomes of
the two species is apparent. This homology suggests that the diploid
species may be one of the parental species of the allopolyploid. But
in many cases, considerable chromosome differentiation mav have
Polyploidy in Plant isreeamg
465
Table 24,3. Some genera which contain allopolyploid species* and one or more
crop species. The crop species themselves may be allopolyploid or
diploid,' Genera like Triticum , Brassica and Gossypium have both
diploid and allopolyploid crop species.
Scientific name Common name Gametic chromosome Cultivated; Wild
number (n j
Avena ssrigosa
Sand oats
7
W
A. bar bat a
Slender wild oats
14
w
A , sativa
Cultivated oats
21
c
A. by z an tin a
Cultivated red oats
21
c
Brassica nig? a
Black sars&on
8(B)*
c
8. oleracea
Cabbage
9(C)
c
8. campestris
Rape, turnip rape
10(A)
c
B, carinata
Abys&inian caboage
17 •
w
3. juncea
Rai, Indian mustard
18
c
3. napus
Rape
19
c
Gossypium arbo •
ream
Asiatic (desi) cotton
13 (As)
c
G. herbaceum
Asiatic cotton
13 (A*)
w
G. thurberi
Wild American cotton
13 (Di)
w
Gw barbadense
Sea island (Egyptian)
cotton
26 (AzDz)
c
G. hirsutum
American upland cotton
26 (AiDi)
€
Hordeum vulgare
Cultivated barley
7
c
H. jubatum
Squirrel-trail barley
14
w
H. nodosum
Foxtail barley
■ 21
w
Medicago hispida
California burclover
7 ■
c
M. lupilina
Biack medic
8* 16
c
M, fa lea t a
Yellow alfalfa
8, 16
c
Nicotlana sy best ns Wild tobacco
12
w
•TV. tomentosa
Wild tobacco
12
w
N, tabacum
Cultivated tobacco
24
c
N. rustica
Cultivated tobacco
24
c
N. bigelovii
Wild tobacco
24
w
N. debneyi
Wild tobacco
24
w
Primus americana
American plum
8
c
P, avium
Sweet cherry
8
c
p. persica
Peach
8
c
P. cerasus
Sour cherry
16
c
P. domes tica
European plum
16
c
Saccharum
officinarum
Noble canes
40
c
5. barber i
Indian canes
41, 45, 46*
c
S sinense
Indian canes
58,62
58, 59
c
; S. spontaneum
Kans (wild canes)
20-64
w
S. robustum
Wild cane
30-74
w
Sorghum versicolo v
S. bicolor
■ Wild sorghum
5
w
Jo war
10
c
S. halep otse
Johnson grass
20
c
Trifolium pr a tense
Red clover
7
c
T* alexandrium
Berseem clover
8
, c
T repens
White clover
16
■' c
T. medium
Zigzag clover
40, 48, 42, 49
■ c
Triticum boeticum
Wild einkorn
7(A)
w
; T. monococcum
Einkorn wheat
7 (A)
c
T. dicoccoides
Wild emmer
14 (AB)
w
T. dicoccum
Emmer wheat
14 (AB)
c
T. turgidum
Solid stem wheat
14 (AB)
c
T. cart hit cum .
Persian wheat
14 (AB)
c
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466
Plant Breeding : Principles and Meh
T. poUmicum
T. durum
Polish wheat
14 (AB)
c
Durum wheat
14 (AB)
c
T. timopheevii
14 (AG)
w
T. amerkanum
14 (AG)
w
T. Apelia
Spelt wheat
21 iABD)
c
T. aestivum
Common bread' wheat
.21 (ABD)
■C
T. macha
21 (ABD)
w
T. vavilovi
21 (ABD)
w
T. eompactum
Club wheat
21 (A8D)
€
T. sphaerococcUm
2! (ABD)
W
* Letters within parentheses denote the symbols used for genomes present
in the species.
C - Cultivated ; W = Wild.
taken place both in the diploid and the polyploid species, and pair-
ing between the chromosomes of these species may be greatly
reduced. Additional evidence on the parental diploid species is
obtained by synthesizing the naturally occurring allopolyploid from
the parental diploid species. The synthetic allopolyploid does often
resemble in many ways the natural allopolyploid species. This has
been clearly demonstrated in Brassica ; for example, B . napus (turnip
rape) synthesized from B. oleracea (cabbage) and B. campestris
(rape) resembles B. napus very closely indeed. But the resemblance
between the synthetic and the natural allopolyploid often may not
be as great, and sometimes it may be very poor. Confirmatory
evidence on parental species is obtained by using biochemical
techniques , such as, electrophoretic patterns of proteins and enzymes,
and from chromosome banding patterns. We shall briefly consider
the possible evolutionary history of some important allopolyploid
crop species, viz., wheat, tobacco, cotton and ' Brassica .
Evolution of Bread Wheat (Triticum aestivum). Evolutionary
history of wheat has been the most extensively investigated,
and is perhaps the least understood. Identity of the diploid
species contributing the three genomes (A, B and D genomes) of
T. aestivum has been investigated by many workers, more notably by
Sears, Kibara and others. It is generally accepted that the A
genome present in diploid wheats is the same or very similar to
those present in tetraploid and hexaploid wheats. Further/ the
B genome of tetraploid emmer wheats is similar to that found
in hexaploid wheats.- This is evident from chromosome pairing
in crosses among diploid, tetraploid and hexaploid wheats. Hybrids
between diploid and tetraploid wheats show about 7 II and 7 1,
while those between tetraploid and hexaploid wheats show about
14 II and 7 1. The above is generally widely accepted. But the
.diploid species contributing the A, B and D genomes are not agreed
upon. It has been proposed that the source of A genome is
T. monococam , of B genome is Aegilops speltoides and of D genome
is Aegilops squarrosa. This is summarised in Fig. 24.5 and in Table
24.4.
However, considerable evidence has been accumulated that
raises serious doubts as to the sources of A, B and D genomes, but
pglyploidy in Plant Breeding 467
Table 24,4. . Sources of A, B and D genomes of Triticum aestivum. common
hexaploid wheat.
Genome
Source 0 diploid
species)
T. monococcum
Aegiiops spehoides
Ae, squarrosa
Evidence
Pairing in F 1 from T. aestivum x T, mono-
coccum and in F z from tetrapioid wheats X
T. monococcum.
Morphological features not contributed
by A genome, chromosome morphology
and chromosome pairing in interspeci^c
hybrids.
Chromosome pairing in interspecific
hybrids, morphology of the amphidipioid
from T. dicoccoidesxAe. squarrosa , pairing
in the hybrid from T. aesttvam x the
synthetic allohexaploid.
tAi^cum mono comum
AEGJLOPS SPELTOIOES
SPONTANEOUS
Chromosome '
■ DOUSlJ'NG
4AEGILQPS' SQUAMOSA K AA BB
<n«7, DD> V
Tetrapioid emmer wheats-
! SPONTANEOUS
' CHROMOSOME
' DOUBLING
aa SB DO
jn=,£h
^MPHlOSPUOfD
Hexaploid wheat
Fig, 24.5. A possible evolutionary history of hexaploid wheat.
M the same time there is no other hypothesis which is more plausible
than the. one outlined above. It ’is, therefore, concluded' that the
evolutionary history of wheat is, at best, not clear and often very
confusing.
Evolution of Nicotiona tobacum. iV. tahacum (n=24) is most
likely an amphidipioid from the cross N. sylvestrisxN. tomentosa ;
both the species are diploid with «=!2. The interspecific hybrids
JV. tahacum X N. sylvestris and N. tahacum X N. tomentosa produce
,12 II and 12 1 at metaohase I. This indicates a homolmrv he.twei&n
450
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458 Plant Breeding : Principles and Methods
chromosomes of tabacum and those of syhestris and tomentosa. The
amphidiploid from the cross N. sylvestrisXN. tomentosa is similar to
N. tabacum in many characteristics, which further supports the above
conclusion. The species N. tabacum has undergone considerable
differentiation during its evolutionary history, mostly due to the
accumulation of gene mutations and, to some extent, due to the loss-
of some duplicated segments of the two gemones.
Evolution of Gossypium hirsutum. The 9 old World the
World species of Gossypium have n= 13 , but the chromosomes of
the Mew World species are smaller than those of th« Old World
species. The other species, G. hirsutum, G. barbadense and G.
tomentosum (wild Hawaii cotton), have n=26 ; w i these species
chromosomes are relatively larger than the remaining 13 - A possible
origin of G. hirsutum is from the cross between Asiatic cotton
G. & arboreum X G. thurberi (American wild cotton), foHowed oy
chromosome doubling of the interspecific hybrid. According to a
more recent scheme, G. hirsutum has originated from the cross
G herbaceum var. africanum X G. raimondii, followed by chromosome
doubling of the Fi.
Evolution of Amphidiploid Brassica species. The origin of ampEidi-
53d Tassica species is presented in Fig. 24.6 based on the famous
U’s Triangle proposed by N. U in 1935. According to thisjeheme,
a ; uncea («=18) is an amphidiploid from B. nigra («— 8) X B.
B ‘ estris (»=10) ; B. napus («= 19) is an amphidiploid from the
B. oleracea (n=9)xB. campestris (n= 10), and cannata
{n = 17 ) is an amphidiploid from the cross B. mgr a (n~ 8) X B, oleracea
(n—9) The synthetic allopolyploids produced according to the above
scheme resemble the natural ampbidiploids, cross easily with them,
and the hybrids between the .synthetic and natural amphidip.oids-
are reasonably fertile.
The determination of parental diploid species of the present-
day allopolyploids presents many difficulties. Some of the major
difficulties are listed below.
1. The parental diploid species may have become extinct, that is*
they may not be present in the nature any more.
2 The parental diploid species may be expected to have differen-
tiated considerably in comparison, to when they entered the
interspecific hybridization to produce the allopolyploid in
question.
3 The allopolyploids themselves may have differentiated to a
great extent from the state when they were first produced.
Thus the genomes in allopolyploids may^ have changed to
various degrees from those of the parental diploids.
In view of these difficulties, the evolutionary history of
allopolyploids can be determined with only limited confidence it
may not be possible to eliminate every doubt about th
the present-day allopolyploids.
polyploidy in Plant Breeding
46S
Fjg. 24.6. The relationship between diploid and naturally occurring amphidi-
ploid species of Brassica. The three diploid species are represented
at the three tips of the triangle ; their amphidiploids are presented
midway between the parental species (and are encircled by two
concentric circles).
’“Letters within parentheses denote the genomic symbols for diploid
and amphidiploid species.
Applications of Allopolyploids in Crop improvement
Allopolyploidy has three major applications in crop improve-
ment : as bridging species in the transfer of characters from one
species to another, in the production of new crop species, and in
widening the genetic base of existing allopolyploid crop species.
Utilization As A Bridging Species. Amphidiploids serve as a bridge
in the transfer of characters from one species to a related species,
generally from a wild species to a cultivated species. The use of an
amphidiploid as a bridging species becomes necessary when the
J^ybrid between the cultivated species (recipient species) and the wild
species (donor species) is sterile. ' The sterility of Fi hybrid makes,
it impossible to cross the recipient species with the Fi, and this does
not permit the transfer of the character from the donor to the
recipient species. In such cases, the chromosome number of the Ft
interspecific hybrid is doubled to produce an amphidiploid which is
generally reasonably fertile and can be crossed to the recipient
synthesized allopolyploids, there-
jcome successful as drops. Thirdly,
superior to the existing' diploid
450
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Plant Breeding : Principle ahd MethdS
species. Progeny from the cross between the recipient species and
the amphidiploid would have the somatic (2/;) chromosome comple-
ment of the recipient species and one genome from the donor species.
As a result, they would be sufficiently fertile to be used in backcross
with the recipient species. From such a programme, alien addition
and alien substitution lines are recovered, which are used m the
transfer of genes, groups of genes or of small chromosome segments'
to the recipient species (Chapter 25). .
An example of the use of an amphidiploid as a bridging species
is the use of synthetic N. digluta (ailohexaploid) for the transfer of
resistance to tobacco mosaic virus from N. sylvestris to N. tahacum
(Fig 24 1) The Fi hybrid from the cross N. tabacumxN. sylvestris
is sterile. Chromosome doubling of the F x hybrid produces the
synthetic ailohexaploid N. digluta, which is reasonably fertile.
N digluta is backcrossed to the recipient species (N. tabacum)
to produce a pentaploid having the complete somatic (2n) chromo-
some complement of N. tabacum and one genome of N. sylvestris.
The pentaploid is sufficiently fertile to be backcrossed to N. tabacum.
Is the progeny N. tabacufn- like plants resistant to tobacco mosaic
are selected " and cytologies l!y analysed. From the backcross
progeny, both alien addition and alien substitution lines can be
recoy-Ted-j Qf the use 0 f ae amphidiploid as a bridging
species are in the cases of transfer of genes from G. thurberi to
G. hirsutum and of chromosomes from Haynaldia villosa to
T aestivum. Many other examples of a similar use of an amphidi-
ploid may be found in the literature. For use as a bridging species,
the features of the amphidiploid are not important ; it has to be only
sufficiently fertile to be backcrossed to the recipient species. Therefore,
this application of synthetic amphidiploids is easy, immediate
and more rewarding than the production of new crop species (in the
run).
Creation of New Crop Species. It was once hoped that allopoly-
ploidy would enable man to create new species at will, and that
these species would be superior to the existing crop .species. This
hope was based on the fact that some of the present-day important
crop species are allopolyploids, and that the existing as well as new
allopolyploids can be synthesized in the same manner as they would
have been produced in the nature. Thus it was expected that
a duplication of the nature’s own methods would lead to the creation
new and superior crop species, as it occurred in the nature.
This hope, however, did not take into account the following
facts ‘.first, allopolyploidy itself has not enabled a species to become
successful as a crop ; in fact, many allopolyploids are weedy wild
species, e.g., S. spontaneum and S. robustum are noxious weeds.
Secondly , the natural allopolyploids have evolved over a lone Period
to their present-day forms. Newly synthesized
fore, could hardly be expected to become
an allopolyploid that would be
Polyploidy in Plant Breeding
47!
'species would have already been produced and refined by the natural
forces. Consequently, the allopolyploids that are not already
existing may be expected to be inferior to the diploid species.
These present a discouraging picture of the possibilities of using new
allopolyploids as crop species, which seems to have been confirmed
by the experience with synthetic allopolyploids.
Tr it kale is the most successful synthetic allopolyploid produced
by crossing wheat (tetraploid or hexaplold) with rye (Fig. 244),
Triticales derived from tetraploid wheats have been the most success-*
ful, but those from hexaploid wheats may also become a successful
crop species. At present, triticales are being, grown commercially in
some parts of the world, e.g., in Canada, and the yields of triticales
are comparable to those of the best wheat varieties. The desirable
features of triticales are that they’ combine the yielding ability and
grain qualities of wheat and the hardiness (tolerance to adverse
environment) of rye. But the development of such superior lines of
triticales has taken 50 years of intensive research. The newly synthe-
sized triticales were of low yielding ability due to high ’sterility, poor
seed set and poor and variable development of grains. Triticales also
show ? cytogenetic and genetic instability due to meiotic irregularities
and produce some aneuploid progeny. In Sweden, the raw triticales
yielded about 50 per cent of the standard' varieties of wheat The
yielding ability of triticales increased under selection to about 90 per
cent of the yield of wheat varieties in 15 years. Extensive breeding
work on Triticale is going on at CIMMYT, Mexico. The strategy
is to (I) produce a large number of triticale strains using different
combinations (varieties as well as species) of wheat and rye, (2T
hybridize these triticale strains, and (3) improve the defects of the
triticales through selection. The results from such breeding pro-
grammes have been spectacular arid have led to the release of
several commercial varieties of Triticale which yield as much as the
best varieties of common wheat.
Some other promising allopolyploids are Raphanobrassica , the
triploid (AAC) obtained by crossing B. napus (A ACC) with
B. campestris (A A), allopolyploid clovers, Festuea-Lolium hybrids and
some species hybrids in Rubus and Jute (Corchcrus In Rophano *
brassica , the breeding objectives are to combine the hardiness of
B, oleracea with quick growth and disease resistance of fodder
radish. The problems of Raphanobrassica are the same as those of
triticales. i,e. 9 low fertility, cytogenetic and genetic instability apd
leafy rape-like plants that do not produce bulbs. There is evidence
that hybridization and selection at the polyploid - level would be
effective in improving Raphanobrassica .
The amphidiplpid. B. napus (AACC) crosses very easily with
B. campestris (A A) to produce the triploid (AAC) which has some
desirable features. The triploid is produced so easily that it tnay be
used as a hybrid variety* a special case of hybrid varieties produced
by crossing Two different species. Yaralakshmi, a hybrid' variety of
Plant Breeding : Principles and Methods
cotton is also an interspecific hybrid between G* hirsutum (American
cotton and G. harbadense (Egyptian cot foil).
Widening The Genetic Base of Existing Allopolyploids. The genetic
base of some natural allopolyploids may be narrow, and it may be
useful to introduce variability in such cases by producing new allo-
polyploids. B. napus is a case in point ; the genetic variability of
this species is narrow and the only recourse available is to
synthesize new allopolyploid B. napm to widen its genetic base.
This is being done by crossing B, campestris (n =10, AA) with
B.oleracea («= 9, CC), the parental diploid species, to., produce
the arophidiploid B. napm (n — 19, AA CC), The two species,
B . campestris and B. oleracea , have to be crossed as autotetraploids;
the cross is very difficult and embryo culture has to be used.
Limitations of Allopolyploidy
1. The effects of allopolyploidy cannot be predicted. The allo-
polyploids have some features from both the parental species,
but these features may be the undesirable ones, e.g., Raphano -
brassica , or the desirable ones, e.g., Triticale .
2. Newly synthesized allopolyploids have many defects, e.g., low
fertility, cytogenetic and genetic instability, other undesirable
features etc.
3. The synthetic allopolyploids have to be improved through
extensive breeding at the polyploid level. This involves consi-
derable time, labour and other resources.
4. Only a small proportion of allopolyploids are promising ; a
vast majority of them are valueless for agricultural purposes.
Thus a costly trial and error has to be done before one is
likely to come across a promising allopolyploid combination
that can be improved through breeding to yield a new crop
species.
SUMMARY
Changes in chromosome number ( heteroploidv ) may involve loss or gain
of one or a few chromosomes ( aneuphidy ) or the whole genome ( euphddy ). A
tyisomic has one additional chromosome (i.e , 2n 4- 1 } ; a monosomic has 2u — 1
and a nullisomic has 2«—2. Aneuploids are generally weak and unstable cyto-
genetically ; monosomies and nullisomics survive only in polyploid species.
Apeuploids occur spontaneously, may be produced in high frequencies from
triploids, autotetraploids, desynaptlc plants, tetrasomic plants and transloca-
tion heterozygotes. Aneuploids are useful in study of the effects of loss or
gain of a chromosome, in locating linkage groups and genes in a particular
chromosome or even in a chromosome arm, in chromosome substitution and in
establishing hemoeoiogy between genomes of an allopolyploid species. Aneu-
p.oid analysis suffers from the difficulty in production and maintenance of
aneuploid lines and the necessity for extensive cytologica! analysis. !
In euploids, the same genome may be present more than twice ( autopoly -
ploidy) or there may be two or more distinct genomes ( ailopolvphidv ). Auto-
polyploids are produced spontaneously, by gamma irradiation, from tissue
culture and by treatment with some chemicals of which colchicine is the most
successful and widely used. Autopolyploids are generally larger In size, have
473
Polyploidy in Plant Breeding
larger cells, stomata* pollen grains and seeds, are slower in growth, later in
flowering and maturity, have thicker leaves, show high sterility and multivalent
formation- Segregation in autopolyploids is very complex. Some of our crops
are autopolyploids, e.g., potato, coffee, sweet-potato, groundnut, alfalfa' and'
banana. Autopolyploidy has the following applications in crop Improvement :
monoptoids are .useful in production of homozygous lines and may be useful
in studies of mutation. Triploid watermelon is seedless and hence desirable,
while triploid so gar beet is high yielding. Tetrciploids may be useful in breeding,
e.g., in banana, in production of interspecific hybrids, e.g., B. ccmpestris X
B. o Israel a (both must be 4/?) and B. oleracla (4m) x B. chinensis, and directly
as crop varieties. e.g* % Double Steel and T.etra Petkus, Giant Berseem etc/ ■ The
limitations of autopolyploidy are higher water content of autopolyploids, high
sterility requiring extensive breeding, difficulties in maintaining autopolyploids
due to cytogenetic and genetic instability, and the inability to predict the
effects of autopolyploidy.
Allopolyploids are produced by chromosome doubling of interspecific
hybrids (. ampnidipldd }. Chromosome doubling may occur in somatic tissues or
may result from production of unreduced gametes which unite to produce an
amphidiploid. Chromosoon doubling is generally induced by colchicine Allo-
polyploids combine some characteristics of each parental species, often are
hardier than parental diploids, some show apomlxis, almost all new allopoly-
ploids sho v variable sterility, often show cytogenetic and genetic instability
and a variable number of multivalents at meiosis. Fertility and bivalent
formations are easily improved by selection. Naturally evolved allopolyploids
show only bivalent formation since they have developed genetic systems that "
prevent homoeologous pairing. e.g. y 5B system in common wheat. Allopoly-
ploidy has played an important role in evolution of plants ; some of the
important allopolyploid crops are wheat, oats, cotton, tobacco, some Brassica
species etc. The most probable evolutionary history of many of the allopoly-
ploid corps has been determined. Allopolyploidy is useful in crop improvement
since the syntheticalfopofypkxds may be used as a bridging species in gene
transfer from a wild species to a cultivated one, as new crop species, e,g. 9
Trlticale . Raphanohrassica etc. and as a source of new generic variation in an
existing allopolyploid, e.g , synthetic B. rtapus, for widening the genetic base of
natural amphidiploid B. nap us. The limitations of allopolyploidy are that its
effects cannot be predicted, raw allopolyploids have many defects which have
to be improved through breeding involving considerable resources, and only a
■small proportion of allopolyploids are of any promise.
QUESTIONS
1. Define heteropioidy. List the various types of changes in chromosome
number. Briefly describe the modes of origin of aneuploids and their
applications in crop improvement.
2. What are the common aneuplold analyses for locating genes on a
particular chromosome ? Give a brief description of these techniques
using suitable diagrams where necessary. : ■
3* Define autopolyploidy. Briefly describe the methods of production of -
autopolyploids and their Importance and limitations in crop improve-
' ment.
4. What is allopolyploidy? Discuss the origin in nature and experimental
production of allopolyploids. '
5. -Briefly discuss the evolution of following crops : (i) Brassica sp.,
(ii) Triricum aestivum , (iii) Nicotiana tabacum , and (iv) Gossypium
hfrsutum.
: 6. With the help of suitable examples, discuss the applications and limi-
tations of allopolyploidy in crop improvement.
7. Differentiate between the following : (i) haploids and monopioids,
(is) double trisomic and tetrasomic, (ill) Homologous and homoeolo-
'gous chromosomes, (iv) autopolyploid and allopolyploid, (v) diploid
, :and dipiosdized species, (vi) aneuploid and euploid, and <vi) poly-
jploidy and heteroploidy*
Plant Breeding : Principles and Methods
Suggested Farther Reading
Allard, R.W. i960, Principles of Plant Breeding. John Wiley and Sons, Inc.,
New York. . # , -
Elliot, F.C. 1953. Plant Breeding and Cytogenetics. McGraw-Hill book Co.,
Inc., New York. T .
IAEA, 1974. Polyploidy and Induced Mutations in Plant Breeding. International
' - ’Atomic Energy Agency, Vienna. . , . 4
t . w r* xr « r£ i Worland, A.J. 1973. Aneupioidy m wheat and its uses m
L s genetS Breeding Institute, Cambridge Ann. Report
1972, 25-65.
Harlan, J.R. and De Wet, M.J. 1975 . On O. Wing, and a prayer : the ongms
of polyploidy, Bot. Rev. 41 : 36!-o9t. #
McNaughton. I.M. 1973. Synthesis and Sterility of Raphanobrassica. Euphy-
tica. 22 : 70-88.
Neilson-Jcnes, W. 1969. Plant Chimeras. Methuen, London. . _
Sears, E R. 1972. Chromosome engineering in wheat, atadier Synsp., University
of Missouri 4 : 23-38.
Simmon ds, N.W. 1979. Principles of Crop Improvement. Longman, London
and New York. - ~
Thompson, T.E, 2977. Haploid Breeding technique for Mx, ^rop Sci. 17 t
Xu unskv, F.J. 1974* The development of Triticale« Adv. Agroa. 26 : 3f5-
* 348!
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CHAPTER 25
Distant Hybridization in
Plant Breeding
In crop improvement programmes, the parents used in hybri-
dization are generally different varieties of the same species. In such
cases, hybridization and production of subsequent generations does
not present any problem (except for self-incompatibility and male
sterility systems). Crosses between varieties of the same species
represent the vast majority of crosses used in crop improvement.
But in many cases, it may be desirable, or even necessary, to cross
individuals belonging to two different species or genera. When in-
dividuals from two distinct species of the some genus are crossed, it
is known as interspecific hybridization . When individuals being cross-
ed belong to species from two different genera, it is referred to as
intergeneric hybridization. An example of interspecific hybridization
is a cross between the cultivated rice ( Oryza sativa) and the wild rice
(Oryza perennis ), while intergeneric hybridization may be Illustrated
by a cross between, wheat , (4x or 6x Triticum) and rye (Secaie
cereale ). Hybridization between individuals from different species,
belonging to the same genus or to different genera, is termed as distant
hybridization \ such crosses are known as distant crosses or wide
crosses . This is because individuals used for hybridization in such
cases are taxbnomically more distantly related than different
varieties from the same species,
HISTORY OF DISTANT HYBRIDIZATION
The first authentic record of a distant hybridization for crop
improvement is the production of a. hybrid between carnation
(Dianthus caryophyllus) and sweetwiliian (Dianthvs barbatm) by
Thomas Fairchild in 1717. Subsequently, a large number of inter-
specific hybrids were produced. Most of the interspecific hybrids
were of no agricultural value, and were of academic interest only.
But many interspecific hybrids, particularly in case of ornamentals,
served as commercial varieties. An interesting intergeneric hybrid,
Raphdnobrassica , was produced by a Russian scientist named
Karpechenko in 1928. Raphambrassica is the amphidiploid from a
cross between radish (R. salivas) and cabbage (B. okracea). RaphanO'*
brassica, was produced with a view to combine the root or radish-
with the leaves of cabbage, but it had roots like cabbage and leaves
like, radish! a useless combination of characters. But the -first
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ioteraeoeric hybrid, and with a greater potential than Raphano •
brassica, was Triticale. Triticale was first produced by Rimpau
in about 1890. Triticale is the amphidiploid obtained from crosses
between wheat and rye. About 50 years of intensive research has
improved the characteristics of triticales considerably ; it is beginning
to become an accepted crop in certain areas, e.g., Canada. At
present, distant hybridization has a definite role to play m crop
improvement programmes : improvement of a few crop species is
based on interspecific hybridization, eg., sugarcan t(S.Offictnarum),
but in most of the cases it is used for the transfer of a few desirable
«enes particularly those for disease resistance. In many crop species,
the genes for disease resistance transferred from the wild species are
extremely useful. Jack Harlan concluded that there are several
examples in which genes from wild relatives stand between man
and starvation or economic ruin.’
BARRIERS TO PRODUCTION OF DISTANT HYBRIDS
The difficulty in the production of distant hybrids varies to a
great extent. In some cases, distant hybrids may be obtained without
an appreciable difficulty, and hybridization may be as easy as inter-
varietal hybridization, e.g., cultivated tomato ( Lycopersicon esculen-
xum) crosses easily with the closely related species (L. pimpi-
nellifolium). But in a vast majority of cases, Fi hybrids may be
-obtained with variable degrees of difficulty, and in many cases
hybrids may not be obtained with the currently available techniques,
£.g., cabbage (S. oleracea) does not hybridize with the closely related
turnip (B. rapa) or rape seed ( B . napus). In general, taxonomic
relationship is a fair guide of the ease with which distant hybrids
may be obtained : closely related speceis may be expected to cross
more easily than those more distanly related. However, this general
relationship is only an indication; specific cases differ to a great
extent and the experimenter has to find for himself how easily the
477
Distant Hybridization in Plant Breeding
species used as female is longer than the pollen tube would normally
grow. Therefore, the pollen tube does not reach the embryo sac,
e.g. 9 in crosses between maize (Z. mays , 5) and Tripsacum sp. (o).
In such cases, the cross may be made by using the species with the
shorter style as female or by cutting off a portion of style of the
parent with the longer style. Pollen tubes fc of polyploid species*
are usually thicker than those of diploid species. When a diploid
species is used as female and a polyploid one as male, the polyploid
tube grows at a slower rate in the diploid style than it would in a
polyploid style. This difficulty may be overcome by using 'the diploid
-species as the male parent or by shortening the style of the diploid
species being used as female. These are some of the more common
barriers to zygote formation ; other barriers may be encountered in
specific cases.
Failure of Zygote Deveplopment. In many cases, fertilization does,
take place, and zygote is produced. But the development of zygete is
blocked at various stages. Failure of embryo development may'
take place due to lethal genes, genotypic disharmony between the
genomes of the two parental species, chromosome elimination,
cytoplasmic incompatibility and endosperm abortion.
Lethal Genes. Some species carry a lethal gene which causes death
of the zygote during early embryonic development. The lethal gene
generally does not have any effect in the species carrying it but
affects only interspecific hybrid embryos. For example, Aegilops
umbellulata has a gene for lethality that has three alleles : U for
early lethality, V for late lethality and / for nonlethality. This lethal
gene is active against the zygotes produced by crossing Ae. umbellu-
lata with diploid (2x) wheats. Similarly, Crepis tectorum has a lethal
gene / which causes death of the embryo in the early stages of deve-
lopment in certain interspecific crosses. Interspecific hybrids in such
cases may be obtained by using a strain (of the species that carries
the lethal gene) carrying the nonlethal allele, e.g., / in Ae . umbellu-
lata, and L in C. tectorum , for making the interspecific crosses.
Genotypic Disharmony between The Two Parental Genomes . In some
cases the death of embryo may occur due to genetic imbalance
rather than the action of specific lethal genes. There is a possibility
that cases of genotypic disharmony may be cases of lethal gene
action as well but due to insufficient genetic analysis the lethal genes
have not been detected. A supposed case of genotypic disharmony
occurs in interspecific crosses in cotton involving Gossypium gossy-
piofdes as one of the parents. * The hybrids from these crosses die in
various stages of development ranging from early embryonic develop-
ment to the reproductive phase of the F x plants. Crosses with
G. davidsoni also show early embryonic mortality. But lethal genes
are known in other interspecific crosses in cotton. In seems likely
that the above cases are also due to lethal genes, but there is nr
clearcut evidence for it.
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47g riant Breeding I Principles and Methods
When the failure of zygote development is due to genetic
inbalance in the hybrid, interspecific
since thev would not survive owing to genetic disturbances. But it
S case, are complicated cases of lethal gene action, interspecific
hvbrids may be obtained by using strains that do not have the lethal
allele. Therefore, in cases where genetic imbalance seems to be the
p : fniinrp of zv 20 tc develop fluent, several strains of each
reaS ^sfho U ld^ bl^WwUhSSh Other. If some combinations
See vSSfe progeny^ while others do not, the case is most likely
Ine of lethal gene action. Different genotypes/strams of a species
^how * considerable variation in ease with which they produce inter-
specific hybrids ; the genetic basis and the mechanism of this varia-
tion is not precisely known.
Chromosome Elimination. In some cases of distant hybridization
.chromosomes are gradually eliminated from the zygote. This
tenemlly Xs not prevent embryo development, but the resulting
embrvo and the F, plants obtained from them are not true mter-
2Sc fic hvbrids since they do not have the two parental genomes
Hu Generally, chromosomes from one genome are successively
Siminated due to mitotic irregularities, and in extreme cases
chromosomes of only one genome may remain m the embryo. Such
embryos are consequently* haploid. Chromosome elimination has
been ^reported in certain interspecific crosses in Nicotiana and
HordeuS, and in crosses between Triticum aestivum (<^ and
bulbosum ($0. When Hordetm vulgare or T. aestivum are used as
male and H. bulbosum is used as female, the embryos obtained from
3, crosses ate generally haploid carrying the _ hap M *
H vulgar e and T. aestivum, respectively. In fact, this cross is the
basis for large scale haploid production programmes in barley.
Incompatible Cytoplasm. In a few cases, embryo development may
be blocked by an incompatibility between cytoplasm of the species
used as female and the genome of the species used as male. Such an
interaction more generally, leads to hybrid weakness and male
sterility in the hybrids. But in a few cases, embryo development may
also be prevented.
Endosperm Abortion. Seeds from a large number of distant crosses
are not fully developed and are shrunken due to poorly developed
endosperm. Such seeds show poor germination, and may .often fail
to germinate. When endosperm development is poor or Jsblocked,
the condition is generally known as endosperm abortion. The endo-
sperm may abort during early embryo development preventing
further development of the embryo, or it may abort at a lat " sta |
allowing an almost complete development of the embryo. Examples of
endosperm abortion ate common in distant hybridization. A familiar
example is the cross between Triticum and Secale to produce Tnti-
cale. Ia this case, the endosperm aborts at a much later stage so
that a snail frequency of viable seeds is obtained. But in the r
H. bulbosum X H vulgare, the endosperm aborts at an early stage so
that viable seeds are not produced.
Distant Hybridization in Plant Breeding 479
In case of endosperm abortion, hybrid plants may be raised by
culturing the hybrid embryo on a suitable culture medium
{Chapter 26). The embryos, in such cases, are removed from young
seeds before endosperm abortion takes place.
Failure of Hybrid Seedling Development. Some distant hybrids die
during seedling^ development or even after initiation of 'flowering
Some interspecific hybrids in Melilotus are chlorophyll deficient and
fail to survive. Such plants may be grown to maturity by grafting
them on to normal plants or by growing them in vitro. Certain
interspecific hybrids in cotton appear normal, but die in various
stages of seedling growth ; some plants die after flowers have been
formed. The mechanisms involved in the failure of seedling develop-
ment may be expected to be of a similar nature as those involved n
the failure of embryo development. Lethal genes, genetic imbalance
and cytoplasmic incompatibility may be involved.
TECHNIQUES FOR PRODUCTION OF DISTANT. HYBRIDS
Many interspecific hybrids may be produced if a sufficiently
large number of flowers are pollinated. However, in many cases
hybrid seeds may not be obtained inspite of a large number of
pollinations. The next step consists of determining the barrier to
production of hybrid embryos and then using measures to overcome
• these barriers. In general, the species with shorter style should be
used as the female parent. Where this is not possible or desirable, a
part of the style of the other parent may be cut off to make it
shorter, e.g., part of the stigma of maize is cut-off when it is crossed
as female with Tripsacum. Different strains within a species show
considerable variation in success in interspecific hybridization
particularly when lethal genes are involved. It may be desirable to
attempt hybridization with more than one strain of each of the
parental species. In some cases, autopolyploidy may be helpful in
interspecific hybridization. B. oleracea ( cabbage) and B. eampestris
<turmp rape) do not cross with each other at the diploid level but
they produce embroys when tetraploid forms of the two species are
crossed; embryo culture has to be used for the recovery of embrvos
Environment also influences the success of hybridization. If
possible, the crosses may be attempted under different environmental
conditions.
When species with different ploidy levels are crossed, hybridi-
zation between them is relatively more difficult than when specie*
with the same ploidy level are mated. In such a case, one of the
following four approaches may be useful. Firstly, they may be
crossed directly; in such a case, the. species with the higher ploidv
level is generally used as the female parent. Secondly, the chromo-
some number of the wild species or j>f the interspecific hybrid (F,)
may be doubled to overcome sterility of the hybrid. Many wild
species of potato have been crossed with S. tuberosum after their
450
480 Plant Breeding : Principles and Methods
chromosome numbers were doubled, while- the latter approach has.
been used in tobacco. In the third approach, chromosome
number of the polyploid species is doubled to obtain a semistenie
interspecific F lf which can be either self-pollinated or backcrossed
to the cultivated species, e.g., 8x N. tahacurn X N. otopnora. Fourthly »
the chromosome number of the higher ploidy level species may be
reduced. This is more difficult than increasing the chromosome
number, but in potato certain combinations of seed parents produce'
35-80% dihaploid (n^2x) progeny. This approach has been
extensively used in potato.
When two species, say A and C, cannot be crossed directly,
a third species, e.g., B, may be used as a bridge species. The bridge
soecies must be compatible, i.e. t able to cross, with both the species
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STERILITY IN DISTANT HYBRIDS
Distant hybrids show variable sterility ranging from complete
fertility to complete sterility. An example of almost complete
fertility is provided by the hybrid between L. esculentum (tomato)
Distant Hybridization in Flam Breeding
m
and L. pimpinellifolium , and that of complete sterility by the sugar-
cane-maize hybrid produced at the Sugarcane Breeding Institute,
Coimbatore, and being maintained by clonal propagation. Distant
hybrids may be classified into two broad groups. The first group
includes those distant hybrids that exhibit at least some fertility so
that they can be maintained by selling, intercrossing among them-
selves or backcrossiog to the parental species. The second group
consists of those hybrids that are completely sterile and have to be
maintained clonally or by doubling their chromosome number .;
the latter increases the fertility and seed set in the amphidiploids
thus obtained. The sterility of distant hybrids may be caused' by
cytogenetic, genetic or cytoplasmic factors.
Cytogenetic Basis of Sterility. Most of the interspecific hybrids
show reduced pairing ; in extreme cases, all the chromosomes may
be present as univalents. The distribution of chromosomes in such
cases is irregular, and it leads to the formation of unbalanced
gametes resulting in partial to complete sterility. Interspecific crosses
also show rings and chains at metaphase I (indicating trans-
locations), bridges and fragments at anaphase I (indicating inversion)
and loops at pachytene (indicating duplication or deletion). These
cytological aberrations also reduce fertility wherever they occur.
Fertility in such hybrids is improved by doubling their chromosome
number, that is, by producing amphidiploids from them.
Stebbins demonstrated that in many interspecific hybrids
sterility may result from small structural changes in chromosomes.
These structural changes would not affect chromosome pairing to
any appreciable degree and would not be detectable at metaphase I.
Stebbins calls these small chromosomal changes cryptic structural
changes . Sterility due to cryptic structural changes would be
difficult to distinguish from cases of genetic sterility.
Genetic Basis of Sterility. Chromosome pairing in some interspecific
hybrids is regular, but they show variable sterility. It has been
suggested that sterility in such cases is due to genes rather than
structural chromosome changes or reduced pairing. For example, Fj
hybrid between foxtail millet, Setaria italica , and its wild relative
5V viridis showed normal pairing and regular formation of 9 bival-
ents. But pollen and ovule sterilities were 70 and 50 per cent,
respectively. Hybrids between varieties of the two subspecies of
cultivated rice (O. sativa ), indica and japonica, show variable sterility.
Oka showed that several genes were responsible for sterility in
indica X japonica hybrids ; these genes acted in complementary
fashion. But there is evidence that the genomes of indica and
japonica rices have differentiated to some extent in terms of chromo-
some structure. Therefore, it is possible that the genes affecting
gametophyte development, i.e., sterility in the hybrids, may, in fact,
be small structural changes in chromosomes.
It is very difficult to conclusively show the existence of sterility
in interspecific hybrids to be controlled by genes. It is likely that
450
482
482 Plant Breeding : Principles and Methods
most cases of supposed genic sterility are due to small chromosome
changes. Such changes would produce in the interspecific hybrids
what Stebbins terms as cryptic structural hybridity , that is, heterozy-
gosity for small chromosomal rearrangements, duplications and
deficiencies.
Cytoplasmic Basis of Sterility. In some interspecific hybrids, sterility
is produced by the cytoplasm. In such cases, the reciprocal crosses
produce fertile hybrids, that is, fertility of the hybrid depends
upon which of the two species was used as female. Hybrids between
certain races of Culex pipiens mosquitoes show reciprocal differ-
ences in fertility, i.e. t the hybrid is fertile when one race is used as
female, but it is sterile when the other race is used as female. Clearly,
in such cases sterility is produced by the cytoplasm. Such instances
of hybrid sterility are also ^known in plants, e.g. 9 Epilobium 9
Oenothera etc.
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CONSEQUENCES OF SEGREGATION IN DISTANT HYBRIDS
Distant hybrids involve two distinct species belonging to the
same genus or to two different genera. These species would differ
for a large number of genes. Therefore, distant hybrids would be
heterozygous for many genes ; the number of heterozygous genes
would be several times the number involved in intervarietal
crosses. Segregation, as a result, would produce a very wide range of
phenotypes in F 2 and the later generations. Many of the segregants
would have entirely new characteristics distinct from those of the two
parental species.
The two species entering a distant cross are expected to possess
somewhat different character combinations of adaptive significance.
Evolution and natural selection would have preserved and developed
those character combinations in these species that make them well
adapted to their environments. When these characters are recom-
bined at random, most of the new character combinations are likely
to be much inferior to the parental combinations. As a result, a
majority of segregants from interspecific hybrids are much weaker
and less adapted than the parental species.
The range of recombinants recovered from the segregating
generations of interspecific hybrids is quite extensive. But the
recombinants recovered represent only a small proportion of the total
recombinants that are possible. Thus recombination in interspecific
hybrids produces only a limited range of variability from the total
range possible. The possible causes of restricted range of recombi-
nants are gametic elimination, zygotic elimination and linkage.
Interspecific hybrids generally show reduced pairing in
extreme cases, no bivalents may be formed. The reduced pairing is
due to chromosome differentiation and shows a lack of homology
between the chromosomes of the two species. In such cases, the
distribution of chromosomes at anaphase I of meiosis is not precise
Distant Hybridization 'in Plant Breeding '
483
and many unbalanced gametes are produced. Even wnere pairing
does occur, crossing over, /.<?., recombination between linked genes,
is drastically reduced. Therefore , segregation for different characteris-
tics does not fit the Mendel ian ratios in from wide crosses. This
may be due to irregular meiosis, impaired recombination and reduced
survival of many recombinant gametes and zygotes .
The male gametes are more sensitive to chromosomal, genetic
nr environmental factors than the female gametes. As a result, many
distant hybrids show a very high male sterility and they have to be
backer ossed with one of the parental species which serves .as the
male parent in the backer oss. But even when the F\ is partially fertile,
the jF 2 is more like the backcross progeny than a true F 2 generation .
This is because the male gametes that are most likely to survive are
those that have the same chromosome complement as that of one of
the parental species or very close to it. In an extreme case, the
surviving male gamete may be identical with one of the parental
species. For example, the surviving pollen from Fi of the cross. T.
aestivum X T. timopheevii is almost exactly like that of T. timopheeviL
However, in many cases, male gametes that are close to, but not
necessarily identical with the parental species in their chromosome
complements do survive and the F 2 shows a much wider range
than the backcross.
APPLICATIONS OF DISTANT HYBRIDIZATION IN
CROP IMPROVEMENT
Many of our important crop species are allopolyploids, e,g. $
wheat, oats, sugarcane, cotton etc. These crops evolved through
distant hybridization, followed by spontaneous chromosome doubl-
ing of the nearly sterile F x hybrids. The chromosome doubling may
have taken place in somatic tissues due to mitotic irregularities pro-
ducing an amphidiploid sector in the plant. Alternatively, unreduced
gametes may be produced due to meiotic irregularities ; when two
such gametes unite, an amphidiploid progeny would be produced.
Colchicine gave man a tool to double the chromosome number of
plants at will, and it was once thought that many new and useful
•crop species would be developed through distant hybridization,
followed by chromosome doubling of the interspecific hybrids. The
■success of this approach was taken for granted since the method was
only to duplicate the process that had already been so successful in
the nature. But experience has had a soberingeffect. Creation of new
crop species is still one of the major objectives of distant hybridi-
zation, but it has found several other, and perhaps more immediate,
applications in crop improvement; eg., alien addition lines, alien
substitution lines, gene transfers, transfer of cytoplasm and, in a few
cases, use as new crop varieties.
Alien-Addition Lines. An alien-addition line carries one chromosome
jpair from a different species in addition to the normal diploid chro-
mosome complement of the parent species. When only one chromo-
some (not a pair of chromosomes) from another soecies is present.
450
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1 . .
Plant Breeding : Principles and Method#
it is known as alien-addition monosome. Alien addition lines have*
been -produced in wheat, oats, tobacco and several other species. The'
purpose of alien-additions generally is the transfer of disease resis-
tance from related wild species, e*g. 9 transfer of mosaic resistance
from Nicotiana glutinosa to N. tabacum. This -addition line had the
full diploid complement of N, tabacum (2»=48) plus a pair of H
chromosome carrying mosaic resistance from N. glutinosa (2n=- 24)
thus the somatic (2n) chromosome number of the alien-addition
line was 50.
Alien-addition lines are of little agricultural importance since
the alien chromosome generally carries undesirable genes. For
example, the addition line in tobacco, mentioned above, had reduced
growth and short broad leaves in addition to mosaic resistance.
Alien-Substitution Lines. An alien-substitution line has .one chromo-
some pair from a different species in the place of one chromosome pair
of the recipient species. Alien- substitution monosome has a single
chromosome (not a pair) from a different species in place of a single:
chromosome of the recipient species. Alien-substitutions have been
developed in wheat, cotton, tobacco, oats etc. In case ,of tobacco,,
mosaic resistance gene N was transferred from N. glutinosa to'
N. tabacum ; the resistant N. tabacum line had 23 pairs' of tabacum
chromosomes and one pair (chromosome H) of glutinosa chromo-
somes. The alien-substitutions show more undesirable effects than
alien-additions, and as a consequence are of no ■ direct use is-
agriculture.
Both alien-additions as well as substiutions are effectively viable
in polyploid species only. Consequently, they are confined to poly.-
ploid crop species, such as, wheat (T. aesttvum ), tobacco (N. tabacum ),
cotton ( G . hirsutum), oat (A. sativa), potato (S. tuberosum) etc. Sub-
stitution and addition lines are produced through the backcross
method. The recurrent parent species (in which the alien chromo-
some is to be added or substituted) is crossed with a related donor
species (from which the chromosome is to be added or substituted).
In the example illustrated in Fig. 25.1, the recurrent parent species
Sis susceptible to a disease, while the donor species R has a gene
for resistance to this disease. The resulting interspecific hybrid is
made amphidiploid by chromosome doubling using colchicine. The
amphidiploid thus obtained is backcrossed to the recurrent, parent
species (species S in Fig. 25.1) to produce a 'progeny carrying the
full somatic (2n) chromosome complement of the recurrent species S
(A A BB) and a single genome of the donor species R (C). This is-
then backcrossed to the recurrent species S ; the resulting progeny
have the full somatic complement of the recurrent species S and a
variable number of chromosomes from the donor species R, Resis-
tant (to the concerned disease) plants are selected and may be back-
crossed to the species S. Resistant plants carrying the somatic (In)
chromosome complement of species S and one additional chrppio-
some are selected ; these are alien-addition ntonosomes* These-
plants w^uld show a single univalent at meiotic metaphase I. Alien-
Distant ftybridtiidtioh in Pfatit breeding
flfedPieNT Species <j> donor species
pio!d>y
genome
gametes
Fi‘ HYBRID
£ (4X )
DISEASE SUSCEPTIBLE
AA SB
x p (2X)
DISEASE RESISTANT
CC
cf
| COLCHICINE
AA BB X AA 3B CC (SX) Q
RECIPIENT AMPH1DIPLOID
SPECIES'S \
AA BB X AA BB C (5X) Q
SPECIES S
(f AA BB X AA SB 4 c (4X4»PEW
CHROMOSOMES
FROM GENOME
C OF SPECIES R)
AA BB+ 1 C
AND
AA BB —1 +1C
I
S&iFlUG
{4X41 CHROMOSOME
FROM C)
(4X-1 41
CHROMOSOME
FROM C)
AA 88 + CC {4X4 1 PAIR OF CHROMOSOMES
AND FROM SPECIES R)
AABB-24CC (4X-1 PAIR 4-1 PAIR OF
CHROMOSOMES FROM
SPECIES R)
48.'
Hybridize the tw>
species R and $
Fi is usually sterile
chromosome numbc
is doubled by colchi
cine.
The amphidiploK
thus produced is tf&r
it is back-cross©'
to the recipient spte
cies S. ^
The progeny is i
pentapioid. with c
single C genome front
the species R and the
* uI1 diploid comply
ment of species S.
The progeny have fui
somatic complement c
S and a few chromo-
somes from R, Diseas
resistant plants selec-
ted and baclccrossecf
to species S. '
Disease resistant
plants similar to
species $ selected.
Alien-addition mono*
some has one extra
chromosome ; sub*
stitution monosom©
has the 2 n number
of S,
2/i+2 disease resis-
tant plants are alien*
addition lines. Re-
sistant plants- with
2n chromosome are
alien-substitution line* :
Fig, 25.1. Production of alien-addition and alien-substitution lines. A* simnlif«vi
scheme based on two hypothetical species S andR; a chromosome fmm
spicies R carrying a disease resistance gene is to be added or sufatfenM
mthe full somatic (2/i) chromosome complement of species $, whfrhiS
susceptible to the disease. F * wmc& IS
486 Plant Breeding ; rrmapees ana Methods
addition monosomes are selfed to isolate resistant plants carrying
one chromosome phir in addition to the full somatic complement of
the species S ; these are alien-addition lines . The hybrid between an
alien-addition line and the recurrent parent species S would, show a
single univalent at metaphase L
Some resistant plants would have the somatic chromosome
number of the recurrent species S. . That is, these plants would carry
the gene for resistance, but would not carry an extra chromosome*.
At metaphase I, such plants may show 2 univalents if one chromo-
some of the recurrent species S was replaced by one chromosome of
the donor species R. These are alien-substitition monosomes. The
alien-substitution monosomes are selfed to obtain resistant plants
with the same chromosome number .as that of the recurrent species
S, but producing regular bivalents during meiosis. These plants
would be alien-substitution lines. Two univalents would be seen at
metaphase l of the hybrid between the recurrent species S and the
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Distant Hybridization in Plant Breeding
487
T. aestivum chromosome 5.B carries a gene, Ph , that prevents pairing
i, between homoeologous chromosomes, e.g., chromosomes 5A, 5B and
I '.f 5D. Fairing with the alien chromosome may be promoted by using
! in the interspecific hybridization a wheat line in which the chromo-
^ some 5B lacks the homoeologous pairing suppressor (i.e. 9 has the
mutant Ph allele in . homozygous state), or which is nullisomic for
5B. Such a technique is useful where the knowledge of the genetic
control of chromosome pairing is sufficiently advanced and suitable
j genetic stocks are available. (3) An alternative technique of a more
I general applicability is to induce translocations using X-rays or
gamma- rays. Many transfers have been made by this method 9
notably in-wheat, and tobacco. But translocations are generally less*
desirable than spontaneous or promoted recombinations since they
tend to transfer chromosome segments that may carry undesirable
^ linked genes. ^ Consequently, many of the gene transfers produced
r by translocations are unsatisfactory for use in the crop improvement
Currently, a technique is being developed in wheat for using such
translocations for isolating lines that have received the gene under
transfer from the alien chromosome segment through recombination.
The techniques used for promoting homoeologous pairing and
translocation are known as chromosome manipulation techniques . It
may be pointed out that in most of the’ cases, interspecific gene
transfers have been made by spontaneous recombination.. Use of
chromosome manipulation techniques may often not be necessary,
and they require considerable cytologies! analysis and are time
consuming.*
Genes affecting a variety of characters have been transferred
from wild relatives ; -these include disease resistance, wide adapta-
tion, improved quality, mode of reproduction, yield etc.
1. Disease Resistance. Disease resistance is by far the most
common characteristic transferred to the cultivated species from
their wild relatives. Some important examples are, rust resistance
in wheat; late blight resistance in potatoes ; resistances to bacterial
wilt, bacterial canker, curly top virus, mosaic virus etc. in tomato;
resistance to many insects and diseases in sugarcane and so on.
There is hardly any crop species which has not benefited from
such a gene transfer. In some cases, the contribution of gene,
transfers has been so great that it prompted Harlan to observe that
*some crops could not maintain commercial status without genetic
support of their wild relatives’. A glaring example is the potato
famine in Europe during 1840s caused by late blight; about one-
million Irish died as a result of this famine. Solatium demissum
provided genes far late blight resistance, which served as a miracle
saver of the potato crop, although only temporarily.
2. Wider Adaptation. Wild species have served as useful
sources of genes for earliness and wider adaptation. Cold tolerance
has been transferred from wild relatives to wheat, onion, potato,
. tomato, -grape, rye and peppermint. Eatliness has been transferred
450
Plant Breeding ' : Principles and Methods
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to cultivated species of Brassica and soybean from their wild relatives.
Wild species of wheat and peas have contributed genes for drought
and heat tolerance. Other similar gene transfers are : salt tolerance
to tomato,, tolerance to calcareous soils in grape, and lack of
photosensitivity to Penntisetum .
3. Quality. There are some notable instances where quality
of a crop has been improved by the genes transferred from its wild
relatives. Genes for increased protein content have been trans-
ferred to rice, soybean, oats and rye. The high protein content
of wild species is generally associated with small seed size ; a
Selection for increased seed size during gene transfers generally
leads to a reduction in the protein content to a level compar-
able to that of the cultivated species. The above mentioned
examples are notable in that they have been achieved in spite
of this difficulty. Oil duality in oil palm has been improved
by genes from its wild relatives. Other examples of genes for
improved quality from wild relatives are : increased soluble
solids (from green-fruited species) and carotenoid content (the
B gene from hirsutum) in tomato ; improved leaf quality in
tobacco (from N. debnevi) ; and improved fiber strength in G.
hirsutum (from the lintless G. thurberi ; the allotetraploid from
the cross G. thurberi x G. arboreum was crossed with G. hirsutum
since the two species are incompatible).
4. Mode of Reproduction. In some cases, genes affecting the
mode of reproduction have been transferred from wild relatives to
ill the cultivated species. Self-incompatibility alleles from' B. Campestris
have been transferred to the self-compatible B. napus for the pro-
duction of hybrid seed. Genes for apomixis have been transferred
to maize (from Tripsacum) and sugarbeet (from wild Beta sp.).
Cleistogamy and self-fertility traits of wild Secalesp. have been
transferred to Secale cereale .
5. Yield. Contrary to the general belief, wild relatives of
many crop species are excellent sources of the much needed ‘yield
genes*. An outstanding example of introgression - of yield genes
from a related wild species into a cultivated species is provided
by the work of Frey and coworkers with oats (Avena sativa).
A . sativa was crossed to A. sterilis ; the resulting Fi was back-
crossed four time to A. savita . In BC 4 and the subsequent selfed
generations, selection for high yield resulted in the isolation of
purelines which gave 25-30% more yield than the recurrent
parent (A. sativa variety used as the parent in the. hybridization
programme). This yield increase is two-times the improve-
ment obtained by intervarietal breeding programmes in U.S.A.
during the period 1905 to 1960. Similarly, we have obtained
equally encouraging results from interspecific hybridization in
Cicer. The Fa from the cross C. arietinum X C. reticulatum
showed very large transgressive segregation for yield and several
yield traits. Selections for increased 100-seed weight, pod number
and seed yield per plant were effective ; F 6 progenies . showing
Jtfztani Hybridization in Plant Breeding 489
upto 40%^ improvement in JOO-seed weight and 30-40%
•increase in seed yield/plant over the C. arietinum parent have been
isolated. Other crops where yield genes have been introgressed from
the related wild sp'ecies are : Vigna , Zea, Arachis 9 vanilla, tobacco
and Ribeii Sugarcane is a special example as all the present-day
varieties have two to three wild species in their parentage. ■
6. Other Characters. Wild relatives have provided genes for.
■several other traits, e.g. 9 dark green colour and excellent leaf
texture in lettuce ; short stature of plants in oil palm and wheat
(from Agropyron) ; and bright red, thin-fleshed red peppers.
Many interspecific hybrids show genetic instability, possibly
doe to increase in the spontaneous mutation rate, e,g, 9
■N. tabacumXN. plumbaginifolia, Z. maysx teosinte, some tomato
hybrids etc. It is just possible that mutations occurring in such
hybrids may be useful in crop improvement.
Transfer of Cytoplasm. Sometimes, it may be- desirable to transfer
cytoplasm of one species to another species. This is achieved by
repeated backcrossing of the Fi hybrid with the species to which
the cytoplasm is to be transferred. The segregating progeny may
be selected for fertility and for resemblance to the recurrent species. •
Five to six backcrosses may be sufficient in most of the cases. The
most common and striking example of the transfer of cytoplasm is
provided by the production of male sterile"' lines. Cytoplasmic male
sterile lines of N. tabacum (tobacco) are produced by transferring the
cytoplasm from N, debneyi , N. megalosipkon or N. bigelovi ; of
T. aestiwm (bread wheat) by transferring the cytoplasm from
T. timopheevii or Aegilops caudata ; other crops where such transfers
have been made are cotton, barley, potato, sunflower and ryegrass.
The foregoing transfers of chromosomes, chromosome segments,
and of cytoplasm are examples of introgression . In introgression,
few chromosomes or genes from a related species are transferred in-
to the normal somatic chromosome complement of another species.
Introgression is believed to have played a role in evolution of
•certain crop species, notably .maize.
'Utilization As New Varieties. In some cases, distant hybrids may
be desirable enough to be used as new varieties. In such a case,
the hybrid should not only be highly productive but must be easily
produced «at a commercial scale. There are, however, only a few
examples of such an application. The hybrid cotton variety Vara-
laksbmi is 'an interspecific hybrid and is very popular. It is a hybrid
between (?. hirustum and G. barbademe : it was developed at the
University of Agricultural Sciences, Bangalore. It yields upto 50
quintals/hectare of long staple cotton which has a spinning count of
upto 80. it is cultivated on an area of about 50,000 hectares.
Subsequently, four more G. hirsufum X <7. harhademe F x hybrids were
released; as commercial varieties; these varieties are ms 156, 1
450
Plant Breeding : Principles ana Methods
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Savitri, Jayalaxmi and K2HC- Sugarcane is a special case since all
the present-day commercial varieties of sugarcane are complex inter-
specific hybrids involving Saccharum offidnanmi (2/2=80), S. spon -
taneum (2/1—40-218) and other species of Saccharum. The commer-
cial varieties of sugarcane have a variable chromosome number
(2/1=100-325), 5-10 per cent of which are from the wild species.
The commercial varieties are not only more resistant to diseases and
pests in comparison to S . officinarum , but are also more productive.
Bajra-napier grass hybrids yield twice as much or more fodder than
the napier grass, the fodder is of superior quality as well. These
napier- Bajra hybrids are becoming increasingly popular. The hybrid
between 1 Brassica napus and B. campestris has 1 many desirable
features and is easily produced. It is likely that this interspecific
hybrid would be used as a hybrid variety in some parts of Europe. *
There are some instances where new varieties have been iso-
lated from the segregating generations of interspecific hybrids. A rice
variety €03 1 was developed from a cross between Oryza sativa
(rice) and O. perennis (wild rice) using pedigree method of selection.
Some rice varieties have been developed from, crosses between the
two subspecies of O. saliva , viz., indica and japoniea . Adt. 27 was
developed from the cross Norm 8 (japoniea) X GEB24 (indica).
Adt, 27 yields 5 tonnes/hectare of medium-fine rice of good cooking
quality. In addition, it is early maturing and photoperiod
insensitive.
Development of New Crop Species. Interspecific hybridization
followed by chromosome doubling of the sterile hybrids, produces
allopolyploids that may serve as new crop species. More commonly,
these' synthetic allopolyploids serve as bridging species in crosses
between two species with, different levels of ploidy. For example, the
hybrid between N. tabacum 4%) and N. gultmosa (2n^24) is
sterile and cannot be used in gene transfers.. But, the amphidiploid
N. digluta (2/1=72) from the cross N. tabacum xN . gluiinosa is
reasonably fertile and produces partially fertile hybrids with
tabacum . Thus the synthetic allopolyploid N. digluta serves as a
bridging species between N. gluiinosa and N. tabacum , and makes it
possible to transfer genes and chromosomes from N. gluiinosa to
K tabacum. Similar examples exist in wheat and cotton. Use of
synthetic allopolyploids as bridging species is the roost valuable and
immediate application of allopolyploidy in crop improvement.
Development of new’ crop species - .by synthesizing allopoly-
ploids is the most fancied goal and in a few cases promising results
have been obtained. Triticale is now a promising crop species
already in cultivation in some parts of the world. The history of
triticales brings out two important points': jirst, a new (raw) amphi*
diploid/allopolyploid is unlikely to succeed as a new’ crop species
and is likely to have several defects and undesirable features, and
second , breeding and selection at the polyploid ■ level would consi-
derably improve the allopolyploid so that its defects may be-
corrected. There are indications that breeding of Raphanobrassica
Distant Hybridization in riant Breeaing
4 jfl
may produce a useful crop species. There are a few other examples
in other crops, e.g., Tritlcum-Aegilops hybrids, Fesiuca-Loiium
hybrids, hybrids in jute ( Corchorus ), Rubus etc.
limitations of distant hybridization
An extensive use of distant hybridization in crop improvement
has been limited by several problems summarised below.
U Incompatible Crosses. Although many distant crosses can be
made with fhe aid of various techniques,- several distant
combinations cannot be crossed. When a cross fails, it is gene-
rally difficult and time consuming to establish the exact cause
of the failure. When the reason for the failure of a cross is
not known, one can attempt to overcome the barrier only by
trial and error. Distant hybridization is the most common, and
perhaps the most useful, at the interspecific level (within the
same genus), but several intergeneric crosses have also been of
great promise, e.g, Triticale , Raphanobrassica etc. Interfamily
crosses are neither possible, and in all probability, nor profit-
able. A technique of somatic hybridization (Chapter 26) is
being developed to produce incompatible distant crosses, but
this technique is yet to be standardised and refined for a
practical application.
2. JFi Sterility , Ft hybrids from distant crosses generally exhibit
variable sterility ; rarely the F 2 is fully fertile, e g., F 3 from
Cicer arietinum X C. reticula turn. Partial sterility of F 3 does
. not present a serious problem, and the P 2 may be" either selfed
or backcrossed to the cultivated species. Fully sterile F/s are-
difficult to maintain, and In some cases doubling of the
chromosome number of the F 2 may not be feasible or desirable.
In any case, F 2 sterility confronts the breeder with a problem
to overcome which he has to resort to specialised techniques
demanding additional resources and time.
3. Problems in Creating New Species . Production of new crop
species through allopolyploidy following distant hybridization
suffers from several problems, e.g., lower economic yields,
poor agronomic characteristics, sterility etc. But * these
characters can be improved through breeding and selection as
is evident from the history of triticales ; however, for this
considerable effort, resources and time are required.
4. Lack of Homology between Chromosomes of The Parental
Species. Transfer of oligogenes from related species is mainly
limited by reduced pairing of the alien chromosome with that
of the recipient species. Reduced recombination may be over-,
come by chromosome manipulation, but this is limited to
polyploid species, and even there considerable cytogenetic
analysis is required to make sure that only a small segment of
, the' alien, chromosome is transferred. In a vast majority of
450
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Plant Breeding : Principles and Methods
casetf, however, the gene transfer has to depend on spontaneous
pairing and recombination. Elaborate techniques are being
developed by E.R. Sears and C.J. Driscoll in wheat to promote
crossing over between the alien chromosome segment carrying
the desirable gene and the chromosome of the recipient
species.
5* Undesirable Linkages . Linkage of the gene(s) under transfer
with some undesirable gene(s) presents a major problem in
using distant hybridization in crop improvement. In fact,
undesirable linkages have prevented commercial exploitation of
many alien gene transfers. In few cases, continued breeding has
been able to break the undesirable linkages, but this demands
considerable time, effort and resources.
6. In many distant hybridization programmes, transfer of recessive
traits and of quantitative characters is not feasible. In fact,
monogenic dominant characters are the most easily transferred,
and such transfers constitute the bulk of interspecific gene
transfers.
7. Lack of Flowering in Fu Fi’s from some interspecific hybrids
fail to produce flowers. Hybrids between species belonging to
the Rhizomatosae and those from the section Arachis of
peanuts do not flower ; all the techniques to induce flowering
in these hybrids have iailtd. Similarly, some interspecific
hybrids in the genus Glycine fail to flower, but when they are
grafted onto G. max they are readily induced to flower.
S. in distant hybridization programmes, it is important to use the
best variety of a crop species as one of the parents. Transfer
of genes from a related species into an agronomically poor
type will necessitate fuither breeding efforts to use that gene.
In many cases, the best varieties of a crop species cannot be
crossed with the wild species; the wild relatives generally cross
more easily with land races (old varieties) than with highly
improved varieties.
9. Dormancy . In some cases, the Fi seeds of an interspecific
hybrid exhibit dormancy. For example, some interspecific
hybrids in Arachis remain dormant for 5-10 years regardless
of the treatments applied for inducing germination. Thus seed
dormancy may render the use of some interspecific Fj hybrids
difficult, and in some cases, it may effectively prevent their
utilization in crop improvement.
Some generalisations about the success of distant hybridization
■are as follows. (1) Polyploids are generally more likely to accept
genes from other species than diploids because polyploids have a
greater genetic buffering i.e. 9 the ability to tolerate genetic distur-
bances due to the presence of additional homologous or homoelogous
genomes. (2) Cross- pollinated species are more likely to benefit
ir cm interspecific gene transfers due to their high heterozygosity. And
Distant fty^ridizgtiqn in Mm Breeding
m
Fig. 25.2. ‘Parbfaani KrantP, a bhiodi (Abelmoschos esculentusYv ariety
developed from the Cross A. zsculentu* y
(Courtesy, Dr. Y.S. Nerkar, Parbhani" ' X A mamhoi
(3) Closely related species are tire most likely to contribute usefijl
genes to a crop species, phieSy due to the high bomofegy between
their chromosomes which facilitates gene transfers.
ACHIEVEMENTS
Distant hybridization has been most commonly used for the
transfer of specific characters, e.g., disease resistance to cultivated
species. Many of the genes for rust resistance in wheat have been
transferred from related wild species. Such examples are many in
almost all the crop.species, and this is where distant hybridization
is likely to find a greater use in the future. Cytoplasm from related
species has also been transferred to produce cytoplasmic male sterile
lines, e.g., in wheat (T. aestivum), tobacco (N. tabacumi), cotton
(G. hirsutum ) etc. But these male sterility systems have not been
commercially exploited due to one or the other problem in hybrid
seed production, and in some cases the alien cytoplasm has undesir-
able side effects, e.g., in tobacco.
A recent example' of transfer of disease resistance from a
related wild species to a cultivated species is found in bhindi.
450
ilient features of segregation in distant hybrids.
3 -addition and alien-substitution lines ? Describe the
their production with the help of suitable examples.
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Plant Breeding i Principles and Metnods
Patbhani Kranti bhindi is derived from the cross Abelmoschusescu-
lentus cv. Pusa Sawani X A belmochus manihot and is completely
resistant to yellow vein mosaic virus. It yields 110-120 Q/ha in Kharif
and 85-90 Q/ha in summer as compared to 105-1 10 Q/ha in Kharif
and 75-80 Q/ha in summer of Pusa Sawani. Its fruits are dark green,
slender and tender ; the variety is becoming popular throughout the
country.
Distant hybridization has been employed for the development
of new crop varieties, but such examples are limited to few crop
species. Sugarcane appears to be a special case where improvement
is based on distant hybridization. But this cannot be applicable to
other crops, more particularly to diploid crops.
Distant hybridization coupled with chromosome doubling is
a potential source of new crop species. This is illustrated by Tri-
ticcde, which also clearly points out that extensive breeding can
•considerably improve inferior raw allopolyploids. Some other possi-
bilities are Raphanobrassica, Tnticum-Agropyron, Triticum-Aegilops
And Festuca-Lolium hybrids etc.
SUMMARY
Hybridization between individuals from different species (belonging to
the same genus or to different genera) is known as distant hybridization ,
Distant or wide crosses may fail due to failure of zygote formation, zygote
development or development of hybrid seedlings. Zygote development may fell
due to fethai genes, genetic imbalance, chromosome elimination, incompatible
cytoplasm and endosperm abortion ; some of these mechanisms may be
responsible for failure of the hybrid (Fi) seedling development. Distant hybrids
show variable male sterility ranging from complete sterility to complete
fertility. The sterility may be due to cytogenetic causes (reduced chromosome
pairing, structural heterozygosity etc.), genetic factors or may be cytoplami-
cally controlled. Genetic sterility is difficult to distinguish from that due to
-structural chromosome changes, particularly cryptic structural changes.
Segregation in distant crosses produces a very wide range of segregants,.
hut this range represents only a portion of the possible segregants. Most of
the segregants are weak and inferior to the parental species. The segregation
patterndoes not fit Mendelian ratios. Often distant hybrids have to be
backcrossed (as female) to one of the parental species due to very high male
sterility.
The most common use* of distant hybridization in crop improvement is
to transfer specific characteristics to cultivated species. Gene transfers are
generally through recombination, but alien-addition and alien-substitution
fines may be produced to permit the use of chromosome manipulation techni-
ques for gene transfer. Chromosome manipulation is achieved by either
promoting pairing of the alien chromosome with the chromosome of the
recipient species, or by producing translocation with the aid of X-rays and
gamma-ravs. Distant hybridization has been used in transfer of cytoplasm,
production of new varieties and development of new crop species. For produc-
ing new crop species, distant hybridization has to be coupled with chromosome
doubling in the Fi interspecific hybrids to produce amphidiploids.
QUESTIONS
1 . Describe in brief the barriers in distant hybridization. How can these
barriers be overcome ?
2. Discuss the salient features of segregation
3. What are alien-addition
procedure for
Distant Hybridization in Plant Breeding
4.
6 .
495
Discuss the variotis applications, achievements and limitations of
distant hybridization m crop improvement.
Discuss the problems of sterility in wide crosses. Briefly explain the
vanous mechanisms involved in sterility of distant hybrids. P “
the following : (i) genetic disharmony! (ii) endo-
sperm abortion, (in' chromosome elimination, (iv) alien-addition
Suggested Further Readings
\llard^R.W.^ 196 0 . Principles of Plant Breeding. John Wiley and Sons, Inc.,
^i29-333. 1976 ’ Geneti ° reS0UrceS ia w{id re!atives «ops. Crop. Sci. 16 :
of resistance
McNauohton, I.H. and Ross, C.L. 1978. InterspeciSc and -interaeneric
hybudizatioc in Brassicae with special emphasis on the Improvement
offorage crops. Scottish Plant Breeding Station. Ann? Report “si
Meyer, V.G. 19/4* interspecific cotton breeding. Econ. Bot 28 : 56-60
Riley, R. and Kember, G. 1965 The transfer of ~i . \ .
wheat. Plant Breeding ts&.^
Simmonds, P r * ac ‘Pl e s of Crop Improvement. Longman, London
Stalker^/T.^SO^. lUriHsation of wild species for crop improvement. Adv-
450
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CHAPTER 26
In Vitro Techniques in
Plant Breeding
In vitro techniques relate to the cultivation^ of plant organs,
tissues or cells in tesst tubes on artificial media. The conventional
breeding methods are the most widely used for crop improvement.
But in certain situations, these methods have to be supplemented
with in vitro techniques either to increase their efficiency or to be
able to achieve the objective which is not possible through the
conventional methods. One example of each situation would illus-
trate the point. Production of pu relines or inbreds involves six to
seven generations of selfiog. Production of haploids through distant
crosses or through pollen culture, followed by chromosome doubling,
reduces this time to two generations. This represents a saving of
4-6 years. The other example is the recovery of hybrids from
distant crosses. Many distant crosses fail because of endosperm
degeneration which blocks the development of the young embryo
and may lead to its death. Removal of embryo at an early stage of
development and its cultivation in vitro ensures a full development
of the embryo and the recovery of hybrid plants from such crosses.
HISTORY OF IN VITRO TECHNIQUES
In vitro techniques were developed initially to demonstrate the
totipotency of plant cells predicted by Haberlandt in 1902.
Totipotency is the ability of a plant cell to perform all the functions
of development which are characteristic of the zygote, i.e , its ability
to develop into a complete plant. Efforts to demonstrate totipotency
led to the development of techniques for cultivation of plant cells
under defined conditions, that is, tissue culture. This was made
possible by the brilliant contributions from R.J. Gautheret in France
and from P.R. White in U.S.A. during the third and the fourth
decades of the 20th century. The first plant from a mature plant
cell was regenerated in 1959 by Braun. Most of the modern tissue
culture media have been derived from the work of Skoog and
coworkers during 1950s and 1960s. The first embryo culture, though
crude, was done by Laibach in 1925 to recover hybrid progeny
from an interspecific cross in Linunt. Subsequently, contributions
from several workers led to the refinement of this technique.
Haploid plants from pollen grains were first produced by
Mahesfawari and Guha in 1964 by culturing anthers of Datura.
This marked the beginning df anther culture or pollen culture for
In Vitro Techniques in 'Plant Breeding
497
production of haploid plants. The technique has 1 been further deve-
loped by many workers, more notably by IP. Nitch, C. Nitch
and coworkers. These workers have shown that isolated miero-
spores of tobacco produce complete plants. Plant protoplasts are
naked cells from which cell wall has been .removed. In I960 ,
Cocking produced large quantities of protoplasts by using cell wall
degrading enzymes. The techniques of protoplast production have
now been considerably refined. It is now possible to regenerate
whole plants from protoplasts and also to fuse protoplasts of
different plant species. It may seem that the in vitro techniques are
still in developing stages. But within a brief period, the tech-
niques have made a great progress. From the sole objective of
demonstrating totipotency of plant cells, the technique's now find
1D both basic and applied researches in a number of
fields of enquiry-. Our concern here is to examine their present
applications and future possibilities for improvement of crop pfopts.
PLANT CELLS UE ° F * N VITR0 CULTIVATION QF
. The technique of in vitro cultivation of plant cells or organs
nifnJTn? d ! VOted to , solv r e two basic problems : first, to keep 8 the
?!.h If /? d organs f / ee / rom microbes, i.e., bacteria and fungi,
and second, to ensure the desired development in the cells and, organs
dfiiSS ThV"(!2 ( “I'r'"' “1i a a » d
ditioes. The first problem can . be eliminated by using modem
1? careful handling during various operations. The
second problem remains the area of active research and -is likely to
some time in the future. The technique of tissue, cell
and organ culture is briefly outlined below. It may be pointed out
that this account is essentially preliminary. p m
Surface Sterilization. The plant part to be cultured in vitro is
known ^ as^ explant. The explants must be surface sterilized to
eliminate bacteria and fungi present on their surface. This is com-
monly achieved by treating them with 1-2 per cent solution of sodium
or calcium hypochlorite or with 0.1 % solution of mercuric chloride,
inc explant is then rinsed several times with sterilized distilled
water to remove the disinfectant. Obviously, this and the subsequent
handling of the explants or cultured cells and organs has to be
done i under aseptic conditions, i.e., environment free from bacteria
and fungal spores.
Nutrient Medium. The medium on which plant cells and
organs are cultured is known as nutrient medium, culture medium or
simply medium. The nutrient medium contains inorganic salts, trace
elements, certain vitamins,, a catbon source (generally -sucrose) and
where needed, giowth regulators, that is, auxins and/or.kinins. There
are many standard nutrient media available, but none of them is
suitable for every purpose. Often, the experimenter has to make
some modifications to develop a medium suitable for his own -needs.
Sometimes, complex organic supplements like coconut water, casein
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To make 1 litre
Stock Solution
{mg! J 000 ml)
Compound
NaH#PO*.HsO
KNOi
(NH*) s SO*
MgSO«.7H»D
Iron compound (330Fe)
CaCl».2HsO
Micronutrients
MnSC4.HiO
B s BOs
2uS0 4 .7H»0
Na»MoO*.2HeO
CuS0 4 .5H»0
CoCla.6HtO
Kl
Vitamins
Nicotinic acid
Thiamine HC1
Pyridoxinc HCI
m-Inositoi
Growth regulators
2 4-D (or any other auxin)
Kinetin (or any other kmin)
Sucrose
Agar (for callus cultures)
t ml or as required
1 ml or as required
20 g
5.5
6 g
The medium may.be solidified by using agar (6g/l) or may be
used as liquid. When liquid media are used, the culture flasks have
to be constantly agitated or shaken on a gyratory shaker to facilitate
aeration. The cells on an agar medium develop into an unorganised
mass of cells known as callus ; consequently they are known as
callus cultures . In the liquid medium, on the other hand, a suspen-
sion of free cells and small cell masses is obtained ; such a culture
is known as suspension culture . The media are autoclaved to free,
them from microbes. Sterilized explants are then placed on nutrient
media ; the operation is done under aseptic conditions.
Environmental Conditions. The organ and cell cultures are main-
fn Vitro Techniques in Plant Breeding 499
Table 26 2. Composition of MS tissue culture medium (Murashige and’Skoog,
Physiol. Plant. 1962. 15 : 473-497)
Compound
Stock solution
(mg! WOO ml)
Amount per Hire
nh 4 no s
1 650 mg
kno 3
1950 mg
MgSO* 7HaO
370 mg
KH,P04
1 70 mg
EDTA, Ferric salt
40 mg
CaCk. 2H2Q
15 g
2.9 ml
Micronutrients
h 3 bo 3
620 1
MNSO».4H a O
2230 |
_ZnS0 4 .7H s 0
S60 !-
1 ml
NaaMoO^HiO
25 j
C11SO4.5H2O
2.5 I
CoCl a .6HsO
2.5 j
KI
83
1 ml
Vitamins
Nicotinic acid
!01
Thiamine HC!
100 1
Pyridoxine HCI
10!*
10 ml
m-inositol
1000J
Growth regulators
2, 4-D (or aoy other auxin)
100
1 ml (or as required)
Kinetin (or any other kinin)
10
1 ml (or as required)
Sucrose
30 g
pH
5.5
Agar (for callus culture)
6 g
Snbculturlng. After a period of time, it may be necessary to transfer
organs and tissues to fresh media. This is particularly tree of tissue
and cell cultures where a portion of tissue is used to inoculate new
culture tubes of flasks. This is known as subculturing . Plant cells
and tissue cultures may be maintained indefinitely by serial sub-
culturing.
Plant Regeneration and Transfer to Soil. The ultimate objective of
the application of in vitro techniques to crop improvement is to
obtain full plants and to transfer them successfully to soil. Produc-
tion of various organs, e.g., root, shoot etc., from cells and tissue
cultures is known as organ regeneration or organogenesis . It is
possible to regenerate plants from tissue cultures of a large number
of species,, including rice ( 0 . saliva ), maize (Z. mays), barley
(H. vulgar e), oats (A, sativa ), sugarcane (S. officinarum), Colocasia
(bund a and arvi), potato (S. tuberosum), pea ( P . sativum\ chickpea
(C. arietinum) and alfalfa (M. sativa). But regeneration in many
species, including most of the legumes, has not been obtained thus
far. Regeneration capacity appears to be genetically controlled, and
it may be possible to improve it by a suitable breeding programme.
In alfalfa ( if. sativa ), two cycles of recurrent selection improved the
regeneration capacity from 12 to 67 per cent
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Plant Breeding' : Principles Wd 'Methods
Transfer of whole plants from test tubes to soil is relatively
easy, The plants may be pretreated prior to .their transfer to soil with
different media designed to make them hardy. Generally, they are
transferred into small pots and are covered with inverted beakers
to prevent excess transpiration. After 3-4 days, the beakers are
removed, but the pots are still kept in diffuse light for 5-10 days.
The plants mav now be transferred to sunlight. Seedling survival
may vary from 50-100 per cent if care is taken.
CLASSIFICATION OF IN VITRO TECHNIQUES
The in vitro techniques are grouped into four categories on
the basis of the plant part used as explant and the type of develop-
ment in vitro. This classification is for convenience and is useful in
discussions on the subject. The four groups of in vitro techniques are
as follows : (1) embryo culture, (2) meristem culture, (3) pollen
culture, and (4) tissue culture.
Embryo culture
In embryo culture , young embroys are removed from developing
seeds and are placed on a suitable nutrient medium to obtain seed-
lings. The embryos generally do not complete development, but
develop into seedlings. Sometimes, embroys from mature seeds may
also be used for embroy culture, e.g., in Iris , orchids etc. So far, it
has not been possible to culture embroys before a certain stage of
development, for example, before the globular stage in the case of
barley. Elaborate media for embryo culture have been devised ; the
tissue culture media may also be used for this purpose with appro-
priate modifications. Suitable techniques are available for embryo
culture in many of the crop species, and may be easily developed
for other species, if needed.
Applications of Embryo Cohere. Embryo culture is being routinely
used in some crop improvement programmes to obtain interspecific
hybrids. It has some other specific uses as well, for example, pro-
pagation of orchids, overcoming dormancy etc. The various appli-
cations ofembryo culture are described below.
Recovery of Distant Hybrids . Distant crosses may fail due to one or
more of several reasons, e.g,, inability of pollen to germinate, failure
of the pollen tubes to grow or, perhaps more commonly, due to the
degeneration of endosperm. When embryo fails to develop due to
endosperm degeneration, embryo culture is used to recover hybrid
plants. Embryo culture has been widely used for this purpose. Some
recent examples are the recovery of hybrids from Hordeum vulgar ex
Secale ere ~!e, Triticum aestivum X Agropyron repens, H. vulgar eX
Triiicum species including T. aestivum etc. Two examples of such an
application of embryo culture will illustrate the point. In case of
Triticale , rare combinations between Triticum and Secale develop
viable seeds; But most of the teiraploid and hexaploid wheats carry
two dominant genes, Kr t and Kr 2i which prevent seed' development
in crosses with Secale. The majority of hybrid seeds are Small*.
In Vitro Techniques in Plant Breeding 502
poorly developed and show very poor germination. Further seeds
are obtained from only 5-10 per cent of the florets pollinated. The
recovery of hybrid seedlings is much greater (50 70 per cent) when
nS^S^ 4 ' dayS 0ld Cary0pses are "-vet 1 and c£rS
Hybrid seeds from T. aestivum x H. vulzare are nnt
but hybrid seedlings have been obtained by embryo culture Hybrid
embryos from i 9-12 days old caryopses were removed and placed on
14-18 days old barley endosperm cultured on a nutrient medium As
many as 10 hybrid embryos may be placed on a single endosperm
A high proportion of hybrid embrovs developed into
they were thus cultured. ' P nto seedJlD g s when
Recovery of Haploid Plants from Interspecific Crosses. When H. vuhart
or T aestivum (used as male) is crossed with Hordeum bulbosum
(used as female), the chromosome complement of H. bulbosum
from the. developing embryo. Most of the seedlings
obtained from such crosses are haploid, having only one set of
chromosomes from H. vulgare or T. aestivum parent. There is some
seeds set in H bulbosum x T. aestivum crosses, but caryopses tend
1° Sf "'?“ K af, ' r I0 , da,S Case »■ »*»«*-» ITJSre
crosses. Embryo culture of 8-10-days old embryos enables the
recovery of a large number of haploid seedlings in both the cases
Propagation of Orchids. Orchids are difficult to propagate since their
seeds lack any stored food and the embryo is virtually naked. In
many orchids, embryo development is incomplete at the time the
™ a J ur , e ‘ J°“ng or mature orchids embryos are removed from
f^ c a m- P aCeC ! l , 0n s . UItab , le nutn 'ent media. The embryos develop
into seedlings either directly or through callus formation.
Shortening The Breeding Cycle. Seeds may take 10-20'days to mature
a er the embryo is sufficiently developed to permit successful culture
k J i r0 - , Th f. embryos do not complete their development in vitro,
but develop directly into seedlings. Thus the next generation may be
grown one or two weeks earlier by embryo culture than from seeds.
»ut this has not become a practical feasibility because of two
reasons : first, embryo culture is an exacting and time consuming
technique and second, excellent green house facilities are a prerequi-
site for such an application of the technique.
Overcoming Dormancy, In some fruit trees, embryos require a period
•or at ter- ripening before germination. Iris seeds may take two to
yws to germinate. This period of dormancy can be effectively
rax«°etc d ^ embryo cubure - Other examples are, Prrnus,
Meristem Culture
. The cultivation of apical meristems, particularly of shoot apical
meristem, is Known as meristem culture. Meristem culture involves
me development of an already existing shoot apical meristem and
ihe regeneration of adventitious roots. It does not involve the
-
502 Plant Breeding : Principles and Methods
regeneration of a new shoot apical meristem. Meristem cultures have
been extensively used for quick* vegetative propagation of a large
number of plant species. Ordinarily, it is not necessary to excise or
isolate the apical meristem for meristem culture. Usually, 2-3 mm
shoot apices containing the shoot apical meristem alongwith several
leaf primordia are used. When the objective is vegetative propaga-
tion, the size of shoot tip used for culture is not important. But when
the objective is to free the stock from a virus, it is essential that the
apical meristem should be excised alongwith the minimum of the
surrounding tissue. t This is necessary in order to get rid of the virus
particles that may be present in the surrounding tissues. The shoot
tip may be cut into fine- pieces to obtain more than one plant from
each shoot-tip. Axillary buds may also be used for meristem culture.
The standard tissue culture media are suitable for this purpose with
some modifications, if necessary.
Applications of Meristem Culture. Meristem culture has many use-
ful applications in crop improvement : clonal propagation, recovery
of virus-free stocks, germplasm exchange and germplasm con-
servation.
Vegetative Propagation , Meristem culture has been extensively used
for vegetative propagation of man}/ crops and fruit trees. Meristem
culture is used for obtaining clones, hence it has been termed as
mericloning, Le. 9 cloning through meristem. A single meristem usually
produces' many piantiets which may again be used for meristem
culture. ' Mericloning is useful in clonal multiplication of vegetatively
propagated crops, e.g., ginger (Zingiber sp.), turmeric (C. domestka )
etc , and of fruit and timber trees, and in. the maintenance and quick
multiplication of some breeding materials.
Recovery of Virus-Free Stocks . Seeds from, virus-infected plants gene-
rally do not contain the virus. Therefore, sexual progeny are usually
virus- free, -except for new infections. But in case of asexually repro-
ducing crops, virus infections spread rapidly. This is because vegeta-
tive propagates from' virus-infected plants contain virus particles-..
But the shoot apical meristems and some young tissues surrounding
them are generally free from viruses. Meristem culture is, therefore,,
useful in recovering virus-free plants from virus-infected plants or
clones.
Many valuable clones in potato (S. tuberosum, ), sugarcane*
(S. officinarum) and other clonal crops have been lost -due to viral
infections. Some valuable clones in potato, sugarcane etc. have been
freed from virus infections through meristem culture. Care must be
taken to remove the apical meristem with as little surrounding tissue
as possible to make sure that virus particles are not present In the
explant. This application of meristem culture is of great value,
particularly in the maintenance of breeding materials and germplasm
which are invaluable for any breeding programme. •
Germplasm Exchange of Clonal Crops And- Fruits. The .commonly;
used propaguies in' most of the clonal crops, particularly in the.castf.
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In Vitro Techniques in Plant Breeding
of those that do not produce seeds, are rather bulky. Further, there
is the necessity of careful quarantine. Seedlings in test tubes obtained
from apical meristems would prove extremely useful in germplasm
exchange of such crops. Such seedlings would be free from patho-
gens and insects, and would be easier to handle. This application is
likely to become considerably important in the future.
Germplasm Conservation . Germplasm conservation in clonal crops,
particularly in case of root and tuber crops, and in trees presents
many problems. Roots and tubers lose viability rapidly and their
cold storage requires large space^and is expensive. Freeze preserva-
tion of meristems and cells in liquid nitrogen at — 196°C has been
suggested for the long-term preservation of their germplasm. This
technique is still in the developing stages ; we shall discuss more
about it later in this chapter.
At present, meristem culture is commercially used for quick
clonal multiplication of many valuable crops. In some cases, it has
been used to obtain virus-free clones. The other two objectives, viz.,
germplasm exchange and conservation, are yet to be used on a
substantial scale.
Anther or Pollen Culture
Haploid plants may be obtained from pollen grains by placing
anthers or isolated pollen grains on a suitable culture medium. The
most suitable stage of the pollen grains for this purpose appears to
be the binucleate stage, that is, after the first mitotic division of the
pollen nucleus. The generative nucleus, i.e., the nucleus that would
have produced the sperms, degenerates. The vegetative nucleus
divides mitotically to produce either an embryoid or a callus mass.
Embry oids are directly produced from pollen grains in Datura and
tobacco. In such cases, the plants obtained from the embryoids are
generally haploid, but some polyploid plants are also produced. In
many species like rice (0. sativa ), barley (//. vulgare), wheat,
cafes etc., pollen grains produce callus from which plantlets 'may be
regenerated ■under suitable, culture conditions. In
ploidy level of plants varies considerably more than m
embryoids are produced. Haploid plantlets have
450
504
Plant Breeding : Principles and Methods
carry;
they i
(idem
Fa ge
cfaror
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wouk
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Chron
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Jfe vJL
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rice, Tanfeng (meaning haploid derived, high yielding No. 1) has
been released for commercial cultivation. In many crops, the
application of this technique is not yet practical because large
numbers of haploid plants are not easily obtained. Therefore, a
wider application of pollen culture in crop improvement depends
primarily upon the development of techniques for quick production
of large numbers of haploid plants.
There is considerable evidence that haploid plants regenerated
from callus cultures show genetic variation, that is gametoclonal
variation. For example, gemetoelonal variation for heading date,
plant height etc. has been reported in rice. Such a variation may
be subjected to selection at the haploid level, and the chromosome
number of the selected plants may be doubled to obtain homozygous
plants. Gametoclonal variation appears to be more desirable than
somoclonal variation because (1) the mutant characteristic is expres-
sed in the R 0 plants, i <?., plants regenerated from the cells in vitro,
and (2) cells having detrimental mutations may . be expected to
regenerate much less frequently in the case of haploid than in diploid
cells. However, regeneration of haploid plants is feasible in fewer
plant species than those from somatic cells.
Tissue and Cell Cultures
Application of plant tissue and cell culture in crop improve-
ment depends solely on the availability of techniques for regenera-
tion of whole plants from them. Both callus and suspension cultures
of many species regenerate whole plants. In some cases, there is
production of somatic embryos, e.g., carrot, sandalwood tree,
Dioscorea (yam) etc., but this is limited to a few species. In many
other species, however, shoot buds differentiate from the cell mass
followed by differentiation of roots, e.g , tobacco, rice, wheat, barley,
coffee ( Coffea arabica ) etc. Regenerated plants often show variation
in ploidy level due to chromosome instability of plant tissue cultures.
Generally, the chromosome instability of tissue cultures increases
with the duration of in vitro culture, while their regenerating
capacity decreases. The tissue culture technique has several
applications in- crop improvement some of which are being exploited
at present.
Applications of Tissue and Cell Cultures. The various applications of
plant tissue and cell cultures are brieSy discussed below.
Clonal Propagation. Tissue culture is well suited for quick vegetative
propagation of plant species. This is particularly true for those cases
Where regeneration of plants from callus and suspension cultures is
relatively easy. It is used for asexual propagation in many species
including some fruit and timber trees. In some cases, it "has been
used for obtaining disease* free and virus-free plants. Quick multipli-
cation of plant cells in vitro often ‘cures’ them of viral infections.
Therefore, plants regenerated from them are generally virus-free.
The major .difficulty in the use of this technique in clonal multiplica-
tion is the occurrence of genetic variation among the regenerated
In Vitro Techniques in Plant Breeding
505
! plants. This variation may arise due to chromosomal aberrations
or gene mutations. This problem can be reduced to a large extent
by using young tissue cultures* preferably during the first few
I subcultures.
i Mutant Isolation. Biochemical mutants are far more easily isolated
j from cell cultures than from whole plant populations. This is because
a large number of cells, say 10 6 - 10 9 , can be easily and effectively
j screened for biochemical mutant cells. Screening of as many plants
j would be very difficult, ordinarily impossible. Biochemical mutants
! could be selected for disease resistance, improvement of nutritional
quality, adaptation of plants to stress conditions, e.g. 9 saline soils,
and to increase the biosynthesis of plant products used for medicinal
or industrial purposes.
^ 1. Somaclonal Variation. Plants regenerated from tissue and cell
| cultures sho* heritable -variation for both qualitative and quantsta-
j tive traits ; such a variation is known as somaclonal variation .
j Somaclonal variation has been described in sugarcane, potato,
tomato etc. Some variants are obtained in homozygous condition in
the plants regenerated from the cells in vitro (R 0 generation), but
most variants are recovered in the selfed progeny of the tissue
culture-regenerated plants (Ri generation). Somaclonal variation
most likely arises as a result of jphromosome structural changes, e.g, 9
\ small deletions and duplications, gene mutations, plasma gene
| mutations, mitotic crossing over and, possibly, transposons. Most
of the variants in tomato were shown to be recessive gene mutations,
and homozygous mutants recovered in the R.<* generation were most
likely produced by mitotic crossing over in the cells before regenera-
v tion of the shoot-tip. Some plasma gene mutations have also been
i recovered. Most of the mutations recovered in tomato were identical
f with the already known spontaneous or induced mutations, but some
mutations were entirely new. Somaclonal variation may be profitably
utilized in-crop improvement since it reduces the time required for
releasing the new variety by at least two years as compared to muta-
; tion breeding and by three years in comparison to backcross method
of gene transfer.
2. Amino Acid Analogue Resistant Mutants. Cereal grains are
deficient in lysine ; maize (Z mays) is also deficient in tryptophan,
while wheat (T. aestivum) and rice (O. sativa) are deficient in threo-
nine. Pulses are deficient in methionine and tryptophan. The
concentration of amino acids in the free amino acid pool of cells is'
primarily controlled by feedback inhibition of the enzymes of ammo
acid biosynthesis. Amino acid analogue- resistant cells may be expected
to show a relatively higher concentration of that particular amino
acid. For example, carrot (D. carofa) and tobacco -(At tabacum)
cell lines resistant to tr^cophan analogue 5-methyl tryptophan
show a 10- 27-fold' increase in the level of trytophan. Similarly, rice
cells -resistant to lysine analogue 5-(p-aniinoethy!)-cysteine show,
■much higher levels of lysine. This techniaue mav nrove useful in the
woul
mom
Mom
in p
Moo*
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Locax
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1 .
Plant Breeding : Principles and Methods
development of crop varieties with a better balanced amino acid
content. But this approach seems to have reached a dead end as the
seeds produced by the mutant lines do not show higher concentra-
tions of the particular amino acids present in the plants. But the
approach may yet hod a use in vegetable crops where the vegetative
parts of plants are consumed.
3, .Disease Resistant Mutants . Many pathogenic bacteria produce
toxins that are toxic to plant cells. Plant cell cultures may be
exposed, to lethal concentrations of these toxins and' resistant
clones isolated. Plants regenerated from these resistant clones would
be resistant to the disease producing pathogen. This technique
should be applicable to all the pathogens which produce the disease
through the action of a toxin. An example of such an application
is ; n the case of wildiire disease of tobacco (;V. tahacum) produced
by Pseudomonas iabaci . Tobacco ceils' resistant to methionine
sulfoximine, which is similar to the toxin produced by the pathogen,
were isolated. Plants regenerated' from, these clones were resistant
lo wildfire disease, although to a somewhat lesser degree. Similarly,
maize (Z. mays) lines having the Texas male sterile cytoplasm are
susceptible to Southern leaf blight caused by Belminthosporium
maydis, which produces a toxin that binds with mitochondria. Maize
ceils resistant to this toxin have been selected, and plants regenerated
from them were resistant to leaf blight caused by //. maydis. These
examples show the potential of the technique. But .much work is
needed before the technique would be of general applicability.
Further, the technique can be applied to those cases only where the
disease is the result of a toxin produced by the pathogen. But many
of the pathogens do not seem to produce a toxin, or the toxin does
not appear to be the primary cause of the disease.
4.. Stress Resistant And Other Mutants . Plant cells resistant to 4-5
times the ’normally toxic salt (NaCl) concentration have been
isolated. In many of the cases, the plants regenerated from
them were also tolerant. to saline conditions. Damage due to water-
logging presumafy occurs due to alcohol accumulation in roots. It
may be expected that mutant 'cells lacking alcohol dehydrogenase
would accumulate very little alcohol and, therefore, would be water*
logging tolerant. Attempts to isolate such cells are being made.
Similarly, attempts are being made to isolate clones' that would
produce more substances of medicinal or industrial valu<
Somatic Hybridization. Protoplasts can be isolated trorn
every plant species and cultured to produce callus. Protoplasts of
two different species may be 'fused with the help of poly
glycol. In some cases, hybrid plants have been obtained afte
of protoplasts, e.g. 9 from fusion of Nicotiana glauca protoplasts
those of N: langsdorfii , two mutant lines of N. tahacum (tobacco) ;
and two sexually compatible species , of Petunia (P, parodii . and
P. hybrid a). It was once suggested that somatic hybrids would be of
great value in crop improvement. But the experience with sexually
Jn Vitro Techniques in Plant Breeding
507
produced interspecific distant hybrids (Chapter 25) is not very
encouraging ; Triticaie is the exception to the general picture.
Further, at present the techniques for selection and multiplication of
somatic hybrid cells* and the regeneration of hybrid plants from
them is limited to a few special cases. It is doubtful that somatic
hybridization would be of any practical value in crop improvement
in the near future.
Genetic Transformation. There is some evidence that gene transfer
may be achieved by feeding cells with DNA in case of eukaryotes,
such as. Drosophila , Neurospora , cultured mammalian cells and in
some plants. Genetic changes may be brought about by DNA or- by
radiation-killed pollen grains. The exact nature of the molecular
events associated with these genetic changes is not known. This raises
the possibility of genetic modification of plant cells with the help of
both homologous (from the same species) and heterologous (from a
different species) DNA. It is also proposed that DNA plant viruses,
such as cauliflower (B. oleraced) mosaic virus and potato leaf roll
virus, plasmids ( e.g Ti plasmid of Agrobacterium) and transposons,
may be used as the carriers of genes for genetic modification of plant
cells. The techniques are yet to be developed for general applica-
bility, and their usefulness in crop improvement is yet uncertain,
although some promising gene transfers have already been made
using the Ti plasmid as vector.
Organelle Transfer. In some cases, it may be desirable to transfer
only organelles or the cytoplasm Into a new genetic background.
This may be achieved through the use of plant protoplasts, Chloro-
plasts have been transferred, and other organelles including nucleus
may be transferred. The technique is yet to be refined for practical
application.
Germplasm Coseirvation. It has been suggested that tissue cultures
maybe frozen and stored in liquid nitrogen at - 1 96°C for long-
term storage of germplasm. This would be of great value in the
conservation of germplasm of those crops which normally do not
produce seeds, eg., root and tuber crops, or where it may not be
desirable to store seeds. For freeze-preservation, the cells are cooled
at a slow rate and are then transferred to liquid nitrogen for storage.
Thawing, of the cells must be very rapid for increased survival. A
cryoprotectant, such as dimefchylsulfoxide (DMSO), is used to protect
the cells from injury due to freezing and thawing. The technique
of freeze-preservation, i.e., cryobiology , of plant cells is still In the
developing stages. In future, it may assume considerable importance
in germplasm conservation, particularly of those species that do not
produce seeds.
ACHIEVEMENTS AND FUTURE PROSPECTS
Embryo culture, is routinely used for the recovery of hybrid
plants from distant crosses-. This is particularly useful where embryos 1
fail to develop due to endosperm degeneration. It is being used
extensively in the extraction of haploid barley { H . vulgar e) from ^
508 Plant Breeding ; Principles and Methods
H. bulbosum < H. vulgare crosses. Barley breeding programmes
based on haploids thus derived have been initiated in several
countries and many varieties have resulted. from these programmes.
Embryo culture is also routine in orchid propagation and in breeding
of those species that show dormancy, e.g., Iris.
Meristem culture and tissue culture are being widely used in
vegetative propagation of many crop species. They are also used
for obtaining virus-free plants from virus-infected clones. These
techniques hold considerable promise in germplasm conservation
and exchange. Tissue culture may prove to be an important tool in
the isolation of biochemical mutants with improved nutritional
quality, resistance to diseases, toleranceto stress conditions etc. Som-
aclonal variation has been exploited develop some sugarcane varities
in India, e.g Co 85001, Co 85003, Co 85006, Co 85007, Co 85008,
Co 8501 1 and Co 85015. Genetic changes may also be brought about
by the use of DNA, DNA viruses and transposons. But these
are only potential applications. Much basic work is needed before
these techniques would become of general practical use.
Haploids are being produced in tobacco, wheat and rice
through pollen culture. These are being used for the development of
disease resistant and superior diploid lines. In China, about two
dozen rice varieties and some wheat varieties have been developed
through this technique Further work is likely to enable the
application of this technique in many other crop species.
The chief problem of tissue and pollen culture is the difficulty
in obtaining regeneration of plants in large numbers. Development
of suitable media and selection for genotypes with higher regenera-
tion capacity may improve plant regeneration in many species. The
future prospects of plant tissue culture and pollen culture would
largely depend upon the availability of techniques for easy regenera-
tion of large numbers of plants.
The in vitro technique is highly sophisticated and requires both
environmental and biological (contamination) control. It is likely
that it may not ever be of general application in crop improvement.
Most likely, a few centres may specialise in these techniques and
would generate materials for the use of plant breeders in crop
improvement programmes.
SUMMARY
The in vitro techniques consist of maintenance of plant organs and cells
on defined media in test .tubes. Tissue culture began with the idea to demons-
trate tofipotency ol plant cells. But now it has many applications. There are
mainly four types of in vitro techniques : embryo culture, meristem culture,
pollen culture and tissue culture. Embryo culture involves removal of young
embryos from developing seeds and growing them on suitable culture media,.
Embryo culture is useful in recovery of hybrid plants from distant crosses,
recovery of haploid plants from certain crosses, propagation of orchids,
shortening of breeding cycle and overcoming dormancy. In meristem culture ,
snoot apical meristem a long with some surrounding tissue Is grown in vitro. It
is used tor clonal propagation and recovery of virus-free plants and in poten-
tially useful in germplasm exchange and long-term, storage, of germplasm
won i<
in one
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Jn vitro Techniques in Plant Breeding 509
through freeze-preservation. Pollen culture is culture of anthers or isolated
nollen grains in vitro. The vegetative nucleus ot pollen grains divides to
oroduc- embryoids or callus masses. Pollen culture is used to obtain hap.oid
nl?n-s which are being used in tobacco and rice improvement. Tissue culture
resists of "'rowing plant cells as relatively unorganised masses of cells on an
coa-ist o (col us culture) or as a suspension of free cells and small cell
mas's™ in 1 liquid medium (suspension ailturcK Tissue culture is use.1 for
*' / ia tnu itiDlication of rnanv species and, in some ca^es, for .recovery of
free it has potential' applications in production of somatic hybrids,
oratnelie and cvtoplasm transfer, genetic transformation and gerniplasm
storage through freeze-preservation.
In vitro techniques are being used in certain special situations. But they
annlication for two principal reasons : first, regenera-
rorhnioue reoures much expertise ana a wen cquippeu ™ j. ■
reason is that the techniques are still in the developing stages and it would be
sonwtime before they become standardised and general, y applicable.
QUESTIONS
1 . Discuss the possibilities offered by in vitro techniques in crop improve-
inerU .
i List the various types of in vitro techniques. Describe in brief any two
” of them and discuss their applications and limitations,
a Define the following : totipotency. organogenesis, expiant, callus,.
cryobiology, mericloning, somatic hybridization, tissue culture. .
a Write short notes on the following : (i) pollen culture, 00 ^sue
* culture, (iii) meristem culture, (iv) embryo culture, and ^regenera-
tion of plants from cells cultivated in vitro. . . .
5. Briefly outline the procedure and technique of in vitro culture of plant
cells and organs.
Suggested Further Readings
TiomwANi S S Evans P.K. and Cocking, E.C. 1977. Protoplast technology
BH in rrelatfon to crop plants : progress and problems. Euphytica 26.343-
360
Collins, G. B. 1977. Production and utilisation of anther-derived haploids in
croo olants. Crop Sci. 17 : 583-586.
Dale P J 1976. Tissue culture in plant breeding. Welsh Plant Brec ing ai
’Ann Report 1976, 101-113.
Evans, D.A., Sharp, W.R. and Medina-Filho, HJP^ 1984. Somaclonal an
gametocional variation. Amer, J. Bot. 71 . ’^774.^ culture
GAMB °Me’th^"sr'Nation^Rls. R, Counci]^ Canada. Prairie Regional Lab..
Kasha K.J. (ed.) 1974. Haploids in Higher Plants, Advances and Potential.
University of Guelph, Guelph. •
Ledoux, L.(ed.) 1975. Genetic Manipulations with Plant Materials, Plenum,
MuRASHWE, Y T. rk 1974. Plant propagation through tissue culture. Ann. Rev.
Plant Physiol 25:135-166^ and Applied Aspects of
REINE M Cell Ttou^anf Organ Culture. Springer, Berlin, Heidelberg and
*cowCRO^ Y W k R.t977. Somatic cell genetics and plant improvement. Advv
StreetCh.e! 1977. Plant Tissue and Ceil Culture. Blackwell. London.
CHAPTER 27
Release of New Varieties
The ultimate aim of any breeding programme is to develop
varieties superior to the existing ones in yielding ability, disease and
insect resistance and other characteristics. The various methods of
breeding are designed to help the breeder in developing such superior
strains. For commercial utilization these strains have to be released as
varieties by either the Central or a State Variety Release Committee
The release of a strain for use as a variety is based on conclu-
sive demonstration of its superiority over the best existing varieties
(included as checks in the evaluation trials) in yielding ability or in
some other feature of economic importance, e.g. t disease resistance
drought tolerance , salt tolerance, etc. For this reason, the strains
are extensively evaluated for their performance, disease resistance
quality etc. in .multilocation trials. The multilocation trials are
conducted under the All India Coordinated Crop Improvement
Projects, which play a key role in testing, identification and release
of new varieties. But before the new strains are included in multi-
location tests, the breeder should evaluate their performance at his
own station to make sure that the new strain is superior to the
existing varieties.
The various activities and operations in the release of a strain
as a variety may be grouped into the following three classes for
convenience in description : (1) evaluation, (2) identification and
(3) release and notification. Following the notification of a variety
its seed multiplication is undertaken.
Evaluation
. Evaluation of a strain for release as a variety consists of various
trials and tests to determine its superiority over the best existing
variety in yield and other agronomic traits, and its suitability for
consumption.. In general, there are seven different types of trials/
tests : (O ration trial, (2) multilocation trials, (3) national trials
n) nn°rf V f X ? a]s i 5 ^ mi £ lklt trialSs disease and jasect tests, and
tests * ali thes e tnais, the best existing variety(ies) is
mcludedasacheck for comparison; in genera], more than one
(often tnree) variety is included as check.
Station Trial. Station trial is conducted by the breeder who has
developed the new stram(s). Such a trial is often referred to as
preliminary yiela trial and may be conducted for one or more vears
The objective of station trial is to make sure thaUfi neSstSS
woufe
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Release of New Varieties 5 1 1
developed by a breeder are superior in performance (at that location)
to the best available variety for the region before they are included
in the trials of the concerned All India Coordinated Crop Improve-
ment Project. This is of great importance because in the trials of
coordinated projects these strains would not only have to compete
with the best existing variety, but also with the new strains developed
by other breeders. Therefore, to avoid disappointment and rejection
in the early stages of testing, the breeder should himself make sure
of the superiority, of the strains he has isolated. In addition, multi-
location trials require considerable human and financial resources,
which are but limited in a developing country like India. It is highly
desirable to utilize these scarce resources for the evaluation of only
promising new strains and not to waste them in the testing of
inferior entries of little agronomic worth.
In station trials of wheat, the plot size is generally 5 mX 1.84 m
•with a spacing of 23 cm between rows (the same spacing is used m
111 the yield trials of wheat). The seed rate is at the commercial
planting rate (100 kg/ha). Plant to plant spacing is not maintained,
but it is important that the stand should be uniform. The number of
replications may vary, but should not be less than 3 to 4. The best
existing varieties are used as checks. Disease reaction and quality
of the new strains are also evaluated. The station trial may be
repeated once to make sure that the new strains are superior to the
existing varieties.
The data from station trials are not required for the inclusion
of a strain in the multilocation trials ; wheat seems to be an excep-
tion where station trial data are proposed to be considered.
Multilocation Trials. These trials are conducted under the respective
All India Coordinated Crop Improvement Projects. The objective of
these trials is to evaluate the performance of newly developed strains
at several locations distributed over a- region. Since the soil and
climatic (agroclimatic) conditions show a large variation from one
region of the country to the other, the country has been divided
info several agroclimatic zones; each zone consists of areas having
similar agroclimatic conditions. The number of zones mid the zoning
pattern vary considerably from one crop to the other. The pattern of
20 nes for a crops is based on several considerations, including . the
distribution pattern of the crop and the area under it m he various
regions. The number of zones for a crop varies from one in the case
ofMowar (sorghum) and bajra, through two in the cases of rajma
(North-Eastern Plains Zone and Central Zone) and safflower (tradi-
tional and nontraditional) to nine in the case of wheat and gram
(Table 27.1) .
The evaluation procedures of wheat are considerably refined,
and they have served as a model for those of sever* .1 other crops.
The description of the nine wheat evaluation zones is summarised
in Table 27.2. In the subsequent sections, the evaluation procedure
of wheat is described in some detail.
512 Plant Breeding : Principles and Methods
Table 27.1. Description of the nine agroclimatic zones for evaluation of wheat
varieties.
WOlllc
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Areas
Northern Hills Zone (NHZ) Hills of Uttar Pradesh, Himachal Pra-
desh, and Jammu and Kashmir.
Northern Plains Zones (NPZ) Punjab. East Haryana, West U.P
Delhi, Ganganagar, Alwar, Bharatpur
(Rajasthan), Gwalior, Bhind, Morena
(M.P.), Uno and Panota Valley (HPP
Jammu (J.&K.) ' * ’
North-Western Plains Zone (NWPZ) Rajasthan (except Kota and Udaipur)
Western parts of Haryana, parts of
Gujarat.
North-Eastern Plains Zone (NEPZ) Eastern U.P., Bihar (except Chhota
Nagpur)
Far Eastern Zone (FEZ) W. Bengal, Bihar (Chhota Nagpur),
Assam, Sikkim, Arunachal Pradesh'
Tripura, Manipur, Mizoram, Nagaland*
Meghalaya,
Central Zone (CZ) Kota and Udaipur - (Rajasthan), Jhmsi
(U.P.), parts of Gujarat, M.P. (except
Eastern -and South-Eastern parts).
South-Eastern Zone (SEZ) Orissa, South and South ‘Eastern M.P.,
Some parts of Maharashtra, A.P, (East
Coast region).
Peninsular Zone (PZ) Karnataka, West A.P., plains of Tamil
Nadu, remaining parts of -Maharashtra.
Southern Hills Zone (SHZ) Hills of Tamil Nadu.
Table 27.2. A summary of the zoning patterns and the • evaluation procedures
used for testing of new strains in different crops.
Zones*
Evaluation
procedure **
Condition of testing
Cereals
9 (NHZ, NPZ, IET, 1 Yr
NWPZ, NF PZ, URT, 2 Yrs
EPZ, FEZ, CZ, AT, 1 Yr
PZ, SHZ)
Wheat
1. Timely sown (15- 25Nov.),
Irrigated
2. Late , sown (15-25 Dec.),
irrigated
3; Rainfed (around 31 Oct.)
1. Rainfed (direct-seeded)
2. Transplanted
3. Direct seeded
4. Deep wafer
5. Saliae/alkalfne soil
6. Aromatic, slender grain
A. Hulled
1. Timely sown, irrigated
:2. Late sown, irrigated
3. Timely sown, rainfed
B«. Hall- 1 ess
4. Timely sown irrigated
5. Late -sown, irrigated
6. Timely sown, rainfed
€; Winter barley (in Hill Zone *
only)
8 (NEPZ, NHZ,
NPZ, CZ,
EZ, FEZ,
SWZ, SZ)
5 (NEPZ,
. NWPZ, CZ.
Diaraland
Zone, Hill
Zone)
IET, I Yr
URT, 2 Yr
AT, iYr
riant breeding : Principles and Methods
Famine Commission (1944), and Ford Grains Policy Committee
(1944).
Quality seeds of vegetables were being imported from countries
like Australia, U.K., U.S.A., Germany etc. upto 1939. In 1945,
some private seed companies, e.g. 9 Sutton’s, Potha etc., began quality
seed production of temperature vegetables in Quetta and Kashmir
Valley ; they formed an Ail India Seed Producers’ Association in
1946. Several private seed companies (see later) were formed during
1950s and 1960s ; during 1970s an estimated 200 private companies
were involved in the production and marketing of quality seeds of
various crops.
Both crop improvement and quality seed production received
impetus after the independence. In 1952, the Indian Council of
Agricultural Research appointed a* Standing Experts Committee on
Seeds which formulated a programme for strengthening the seed
production and distribution systems. As a consequence, the Central
Government provided financial assistance to the states for this
purpose, and in 1956-57 the ‘State Seed Farm Project’ was initiated.
Under this programme, states began production foundation seed on
their State Seed Farms, but this programme was confined to cereals .
only. In each development block a 10 ha seed farm was established
during the first five year plan ; these farms were 4,328 in number,
covered an area of 43,280 ha and involved an expenditure of 18
crore rupees.
In 1959, the Agricultural Production Team, headed by
Dr. Johnson recommended that uniform standards of seed certi-
fication and seed laws should be brought into force and that each
state should establish seed testing laboratories. The Planning Com-
mission appointed a Seed Multiplication Team to review the various
aspects of seed programmes ; this team made several valuable recom-
mendations. Similarly, the ICAR set up a committee in I960 in order
to suggest ways for developing a strong seed production programme;
this committee suggested the establishment of central and state
agencies for the production of foundation seed, the establishment of
independent seed certification agency, the enactment of a national
seeds act and the creation of agencies for its enforcement, and for
stimulating the development of the private seed industry. Based on
these recommendations, the National Seeds Corporation (NSC) was
established in 1963, and the Indian Seed Act was enacted in 1966.
The Government of India set up a Seeds Review Team in 1968 ; the
team toured several foreign countries and made some far-reaching
recommendations. One of the recommendations was that Agricultural
Universities should also be involved in foundation seed production.
The seed industry developed fairly rapidly with the establish-
ment of NSC. Both the quantity of quality seeds produced and the
number of crops covered has rapidly expanded. For example,' the
total quantity of quality seed produced in 1966-67 was only 36,080
quintals as compared to 48.46 lakh quintals in 1984-85 (Table 28.1).
450
Quality Seed : Classes , Production Practices and Maintenance 527
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different types of seeds produced. The different types of seed denote
different stages in multiplication of a variety, and different levels of
genetic and physical purity as well.
HISTORICAL
The procedures for seed production and processing and the
standards for seed certification developed slowly with the realization
of the importance of quality seed in agriculture. Most likely, seed
certification began in Sweden during the last quarter of the nine-
teenth century. In i 886, the Swedish Seed Association was formed ; it
undertook the production and distribution of quality seeds of
improved varieties of niainly forage crops. Towards the end of the
19th century, Dr. E. Helve established a seed testing laboratory in
Denmark in order to conduct seed tests for certification, in 1917,
Dr. J.W. Robertson, a Canadian scientist, proposed the production
of foundation seed, which is now a crucial stage in quality seed
production.
In 1919, the International Crop Improvement Association
(renamed as Association of Seed Certification Agencies in 1969) was
formed. This Association has been crucial in the development of the
procedures and the standards for quality seed production and seed
V certification. In 1946, the Association clearly defined the four classes
m (breeder seed, foundation seed, registered seed and certified seed ;
1 see later) of quality seeds in relation to forage crops. Later, in 1968,
1 the Association proposed that the same four seed classes be adopted
for grain crops as well. The procedures and the minimum quality
standards for seed production, defined and published by the asso-
ciation in 1946, have been used, with appropriate modifications by
U.S.A., Canada and many other countries, including India.
DEVELOPMENT OF SEED INDUSTRY IN INDIA
In India, new improved varieties of some important crops,
notably wheat, were developed during the first two decades of the
twentieth century. Efforts were made by the State Departments of
Agriculture and some other agencies, e.g., some princely states, to
disseminate the seeds of these improved varieties among the farmers.
For example, the State Department of Agriculture, U P., produced
and distributed more than 1 5. COO quintals wheat seed. A limited
facility for seed test was available at Kanpur. During 1920V the
Government of U.P. initiated a project for establishing seed godowns
in every subdivision ( tehsil ). Clearly, the then government was
amply aware of the importance of quality seeds and efforts were ini-
tiated for their production and distribution.
In 1925, the Royal Commission on Agriculture reviewed the
production and distribution of seeds in India, it made several notable
observations and valuable recommendations. Other committees/
persons who reviewed and made recommendations for the seed situa-
tion in India are as follows : Sir John Russel (1937), Imperial
Council of Agricultural Research (1940), Dr. Barnes (1944), the
CHAPTER 28
Quality Seed : Classes, Production
Practices and Maintenance
The primary objective of plant breeding is to develop superior
varieties. The benefits from superior varieties can only be realised
when they are grown commercially on a large scale. Seeds of im-
proved varieties must be multiplied at a large scale in order to fnake
them available to farmers for commercial cultivation. Here, seed
means seed or any other propagating material used for raising a crop ,
and it does not mean the particular plant pari botanically known
as seed. For example, grain produced for consumption is not seed ;
only grain produced for raising a crop would be called seed. On
the other hand, potato tubers produced for planting a new crop are
known as seed potatoes. During the multiplication of varieties for
use as seed, it is essential that genetic purity of the variety must be
maintained. If the genetic purity is not maintained, superiority of
the variety is likely to be. lost. In addition, for best results the fanner
should use new pure seed every few years fn case of self-pollinated
crops, and every year (hybrid varieties) or every few years (compo-
site and synthetic varieties) in case of cross-pollinated crops. This
would require the maintenance of seeds of superior varieties in
genetically pure state, which would be multiplied every year to
supply new seed to the farmers
The value of pure improved seed can hardly be overemphasis-
ed. The yielding ability and other characteristics of a variety are
governed by its genotype, provided the environment is not limiting.
Whether a farmer produces a good crop or a bad one with the help
of all the inputs at his command largely depends on the variety he
has chosen for cultivation. Pure seed of a superior variety would
mean a good crop harvest, while contaminated seed may lead to
crop failure. The seed may be contaminated by weeds, diseases or
insects and may have poor germination. Use of such seeds would
lead to poor crop stands, disease and pest problems and finally poor
yield. Clearly, the seed of new varieties should reach the farmers
in a pure and healthy state. To ensure this, elaborate seed program-
mes exist in most of the countries. India also has a well organised
seed production programme in the form of National Seeds Corpora-
tion (NSC), State Seeds Coiporafion (S SC) and S f ate Seed Certifica-
tion Agency (SSCA). These organisations are r estvnsib!e for . seed
certification and distribution, in essence for production and distribu-
tion of high quality seed. In a seed production programme, there are
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plant Breeding : Principles and Methods
524
f a o-i culture of the State (Chairman) ; (2) Director, State Seeds
of Agncultur Director State Seed Certification Agency ;
Corporation, (3) Dmectoj, of the Statc ; ( 5 ) Joint
(4) Addition 8 ^ ^ Directors of Research
^StrinTS’ of the State In addition,
t nfrectors of Agriculture of the State ; (8) Officers-m-
( rlSS of all Regional Agricultural Testing and Demonstration
Ch&rg , /q\ Asroao!nists 9 Pathologists and Entomologists
fSuAitouST “ Agricultural in .be state
of ail Ag . noted that the persons listed under items
;“8 andT^in^ kTno. ^members «f the committee, bur Urey
£e invhed to assist the committee in its deliberations.
M0U WhenTn entry is identified by the workshop of the concerned
• Y the breeder begins seed multiplication of that strain in the
project, the breeder o g produced by the breeder after a
following crop seaso . released as a variety is termed as
strain as breeder seed once the
the stock seed. T « < not :g e( 4 the crop season following
identified ... am » ^a." l .^°rT.; STuJsed variety is pro.
notification, ^ season following the release of a variety,
duced. m the se con P duced Thus, the farmer is able to
certified seed of the variety l P released variety for commercial
obtam certified seed of y season following the release and
cultivation only m the Uuro the seed 0 f such a variety
notification of the variety. release (in the crop season
reaches many farmers even before irts release i. kgh
following identification of the strain oy^ Ae seed from
Minikit Trials ; ma “ y d ° th . d the Minikit Trials) and so on in the
these farmers ; who ^^ st an d ing new variety is likely to reach
next t^y^veJ ^fore its certified seed has been put on sale.
lomoy- Ves?a“pSc.s Save been disenssed in some deusl.fi tb.
following chapter (Chapter 28).
SUMMARY
developed in station trials Out-
A breeder evaluater new strains by P the A11 India Coordinated
standing strains are include 1 b a sis. In the first year, the new strains
Crop Improvement Projects on a zo - on Trial- Outstanding entries are
(called entries) are tested in I , (URT) in the second vear. In the third
promoted to Uniform Regional Trials afe retained in URT and are
year, outstanding entries fro - ^ respective crop improvement projects,
also included is, of URT) the entry
Based on the data of the ^ r prereleases multiplication at the annual
mav be proposed for ideaf i Scat ion t i eC #s In the fourth vear, the entries
workshop of the respective emordma ted P J .; e Research Trials and Model
identified for release are f™' aa * a l. carried out from the first year m lBT to
f '°” “
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Release of New Varieties 5?3
Release of A Variety by State Variety Release Committee
In addition to the Central Variety Release Committee each
state has. its own variety release committee. The State Variety
Release Committees usually require a separate set of trials conducted
at the Regional Research Stations of the state, in addition to the
data from coordinated multilocation trials. The procedure for
testing of strains in the State of Uttar Pradesh is briefly summarised
below.
First Year The new entries are evaluated under one of the following
three conditions : {;) timely sown, irrigated ; (ii) late sown, irrigated'
and (m) timely sown, rainfed. 5 9
Note. The plot size for these trials is 5 X 1.84 m and the number of
replications is 4 to 6. Outstanding strains are promoted to th*
second year in trial. v
Second and Third Years. The promising entries are evaluated under
the same condition under which they were evaluated in the first year
of trial. In addition, the strains are also included in an Agronomic
Trial designed to determine the optimum date of sowing and the
optimum level of fertilizer application. 6
Note. After the above evaluation, a strain may be proposed for
release by the State Variety Release Committee.
The requirement of three-year evaluation at the Regional
Agncu.tural Testing and Demonstration Centres of the concerned
state for release of varieties by a State Variety Release Committee
has some merit. The various locations within a state may not have
been adequately represented in the evaluations under coordinated
P/W „ Ia Edition, parts of a single state may be included in
different zones of evaluation m the coordinated projects. Thus eva-
luation at a number of locations within, a state gives a more clear
f. 1 ' Jr tiie ^ptation of a variety in different zones within a
state. However, it does delay the release of a variety by a couple
fjstZinz vStoT* S ° metimeS> hiDder the Spread of aa demise
ho „J/Ti ety , rel e . ased by a State Varie{ y Mease Committee must
th , e Mmistr y of A sr i cu it ur e, Government of India before
can be produced Further, only those varieties are
notified by the ministry which have been identified for prerelease
A 1ml vlrZf V h f W0r ^ h °pafthe concerned coordinated project
a varietv V wh/rb R h SaSe ?°^ mUtce ,’ hcwever ’ »ay release a strain as
ty whicti has not been identified by the workshop of the
eSted Pr ,° jeCt ( ,° r CVen SUch «™i»» which ^vere' not
be norified 1 ° rdln | te d Projects). But such varieties cannot
ed, and ceitified seed of such varieties cannot be produced.
Constitution of The State Variety Release Committee The State
Variety Release Committee consists of the following (l> Direct
522
Plant Breeding : Principles and Methods
notification of the variety* A variety must be notified by the
Ministry of Agriculture, Government of India, before its seed can be
certified.
The various trials conducted for evaluation of wheat strains
before release are summarised below yearwise.
First Year
1. Initial Evaluation Trial (IET) (timely sown, irrigated ; late sown,
irrigated ; and timely sown, rainfed).
2. Disease tests.
Note. Outstanding entries promoted to URT, the rest are rejected.
Second Year
1. Uniform Regional Trials (URT) (timely sown, irrigated ; late
sown, irrigated ; and timely sown, rainfed).
2. Disease Tests.
3. Quality Tests.
Note. Outstanding entries retained in URT and included in Agro-
nomic Trials ; poor entries rejected.
Third Year
L Uniform Regional Trials (as in the second year).
2. Agronomic Trials (dates of sowing ; and number of irrigations :
one, two and adequate).
3* National Trials (top-ranking entries in URT from each zone
pooled ; serves as IET for other zones).
4. Disease Tests.
5. Quality Tests.
Note . Outstanding entries proposed for identification ; proposals
for identification submitted by the concerned breeders at the annual .
workshop of the concerned coordinated project. Remaining entries
are rejected.
Fourth Year
1. Adoptive Research Trials
2. Model Agronomic Trials
3. Disease Tests
4. Quality Tests .
Note. The breeder may propose for release of the variety. The
proposal is considered by the Central Variety Release Committee.
If the variety is released, the Ministry of Agriculture and Irrigation,
Government of India issues a notification for seed multiplication and
distribution of the variety.
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Release of New Varieties
521
Table 27.3.. Proforma for the submission of proposal for the notification of a
variety under Section 5 of the Seeds Act, 1966 (180 copies of this
proposal are submitted to the Ministry of Agriculture and Irriga-
tion, Government of India)
1. State :
2. Crop :
3. Name of the variety under which
released :
4. Year of release :*
5. (a) Parentage with detail of its
pedigree :
(b) Source of material in case of
introduction :
(c) Breeding method :•
(d) Breeding objective :
6 . State the varieties which most
closely resemble the proposed
variety in general characteristics :
7. (a) Breeder/Institute responsible
for maintaining breeder’s
stock : ^
(b) Quantity of breeder’s seed of
the variety available (in kg) :
8. Description of variety /hybrid :
9. Description of the .’parents of the
hybrid. Is there any problem of
synchronization ? If yes, method"
to overccme it :
10. Described t least two identifiable
and distinguishable morphological
characteristics of the variety. In
case of hybrid, please describe
also at least two identifiable and
distinguishable morphological
characteristics of both the
parents :
11. Maturity group (early, medium
and late, wherever such a classi-
fication exists) :
12. Disease and pest resistance (give
details of any resistance to pests
or diseases including races) :
13. Recommended ecology ;
14. Yield (in kg/hectare)
(a) Commercial product :
(b) Seed :
15. Current approximate percentage
of the area of the crop (kind)
under this variety in the State ;
16. Recommendation of the Ail India
Workshop about ihe variety %
Signature of the Chairman /Convenor,
State Seed Sub-Committe©
Name
■ Designation
r
520 Plant Breeding ; Principles and Methods
identification are very rigid. A cotton variety (other than hybrid)
must outyield the check variety by 10 20%, while a hybrid must have
30% superiority in yield over the check for being identified ; further,
they must be at least comparable to the check in disease and insect
resistance. These criteria are fixed by the workshop of the concern-
ed coordinated projects and reflect the experiences of the workers
with the concerned crops. As a result, they are not fixed entities,
and are likely to vary as per the needs of the nation as well as the
experiences of the concerned workers.
In addition, one of the essential requirements for identification
of a strain is that the breeder must be able to spare 10 Q seed of
the proposed strain. This seed is supplied to State * Farms Corpora-
tion of India (SFCI) for prerelease multiplication as well as for
distribution for adoptive and minikit trials.
Release of A Variety by The Central Variety Release Committee
After identification, a variety is tested for at least one year
in Adoptive Research Trials. During this period, disease tests and
quality tests are also conducted. Based on the data from adoptive
■ trials and disease and quality tests, the breeder who has developed
the concerned strain submits a proposal for its release as a new
variety for consideration by the Central Variety Release Committee.
The proposal is prepared according to a prescribed proforma (Table
2.73) giving full details about the pedigree and the breeding method
used f or developing the strain and the performance of the strain In
the various trials, disease and quality tests, adoptive trials etc. The
breeder usually gives the .variety a name or he may give it a number,
which is included in the proposal.
The Central Variety Release Committee consists of the follow-
ing : (1) Deputy-Director * General (Crop Sciences), ICAR* New
Delhi (Chairman) ; (2) Production Commissioner, Government of
India ; (3) Project Director/Project Coordinator of the respective
;rop improvement project ; (4) Principal Investigators for all the
lisciplines of the concerned coordinated project ; (5) Directors of
\gricuituio of all the states (the presence of Directors of Agriculture
of the states withm the zone for which a variety is to be released
is essential) ; (6) a representative of the National Seeds Corporation;
(7) Director, High Yielding Varieties, Ministry of Agriculture
and Irrigation, Government of India ; (8) Deputy Secretary (seeds),
Ministry of Agriculture and Irrigation, Government of India ;
and (9) Project Coordinator, All India Coordinated Agronomic
Research Project. *
After a variety has been released for a zone by the Central
Variety Release Committee, the Director, High Yielding Varieties,
Ministry of Agriculture and Irrigation, Government of India,
notifies the concerned authorities of the states within that zone for
seed multiplication and distribution of the variety ; this is known as
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Release of New Varieties
for various uses. For this reason, quality tests are generally done
io certain specialised laboratories well-equipped for .the purpose. The
Quality tests are generally carried out on all the entries included in
URT/CVT.
Quality tests of wheat are done at the quality laboratories of
IARI, New Delhi : PAU, Ludhiana ; and G.B. Pant University of
Agriculture and Technology, Pantnagar. Similarly, quality of pro-
mising new cotton strains is evaluated in Cotton Technologies
Research Laboratory, Matunga, while the quality of promising new
strains of jute is tested in Jute Technological Research Laboratory,
Calcutta. The following tests are conducted to assess the quality of
wheat strains : (1) protein content, (2) Polshenke test, (3) chapati
making quality test/ (4) bread making quality test, (5) mixograms,
(6) alveographs, and (7) sedimentation value.
Identification of Entries for Release
Outstanding strains are identified for release as new varieties at
the annual workshop of the coordinated projects on the respective
crops. The proposals for identification in some crops are consi-
dered in an open forum by the participants in the workshop, while
in some other crops, e.g., wheat, they are examined by an Identifica-
tion sub-committee, which ordinarily has the following composition :
(1) Deputy-Director General (Crop Sciences), ICAR. New Delhi
(Chairman) ; (2) the Project Director/Project Coordinator of the
concerned Crop Improvement Project ; (3) Principal Investigators
for the different disciplines, e.g., Agronomy, Pathology, Quality
Entomology, Physiology etc., of the concerned coordinated project’;
and upto 5-6 persons actively involved in the project.
Proposals for the identification of entries are prepared by the
respective breeders who have developed the concerned strains. They
are prepared according to a prescribed proforma and include the
performance of the entries in URT for at least two yeans, data
from pathological, entomological, quality tests etc,, and those' from
Agronomic Trials.
An entry considered suitable for release as a variety by the
concerned workshop is said to have been identified for prerelease
multiplication ot simply as identified. The criteria for identification
of enteries vary considerably from one crop to the other. In wheat,
an entry may be identified if it falls into one of the following two
categories : (1) an entry significantly superior to the check in yield
and comparable to the check in disease resistance, and (2) an entry
comparable to the check in yield but consistently and markedly
superior to it io disease resistance. However, an entry immune to
diseases but significantly inferior to the check in yield is not identified
since yield is the most important breeding objective in any croo. In
comparison, entries of oilseed crops must show 10% superiority in
yield over the check before they can be identified by the workshop.
Cotton provides an extreme example where requirements for
518
Plant Breeding : Principles and Methods
T I
f !
400 places within a zone. In wheat, five kilograms of seed of each
variety is planted without any replication ; the agronomic practices
are those for which the variety is identified for release. The objective
of Minikit Trials is to popularise the new variety among fanners of
the zone. It also serves anothe purpose ; the seed of a good new
variety reaches the farmers one year earlier than when its certified
seed would be available in the market.
Disease And Insect Tests. Entries are evaluated for disease and insect
resistance throughout the period of testing, that is, during IET as
well as URT. Disease and insect resistance is tested both under
natural epidemic as well as artificial epiphytotic (epidemic) condi-
tions. The disease reaction tests for different diseases of various
crops are carried out at different places where epidemics of the
concerned disease occur regularly. The places where a disease occurs
at a high intensity, that is, the disease occurs in an epidemic form,
every year are- known as hot spots for that disease. The hot spots
for some of the common wheat diseases are as follows : (1) black
rust (stem rust), Peninsular Zone ; (2) leaf rust (orange rust), all
zones ; (3) stripe rust (yellow rust), Northern Plains Zone ; and
(4) Karnal bunt, Ludhiana (Punjab). Dhanla Kuan (H P.), Pant-
nagar (U.P.) and Gurdaspur (Punjab). The disease reaction of
various entries is evaluated by pathologists at the respective centres.
In addition, the disease reaction is monitored throughout the zone
wherever the trials are conducted. For testing disease resistance
under artificial ephiphytotie conditions, specialised laboratories have
been established. Resistance to wheat leaf and stripe rusts is tested
(under artificially created epidemics) at Simla, while that to stem
rust is tested at Delhi, Indore and Mahabaieshwar centres.
The value of disease' tests cannot be overemphasized. In an
extreme case, susceptibility of released variety to a disease may spell
disaster to the. farmer, and sometimes, may even cause health hazards
to the consumers as well The early hybrid bajra varieties (e.g., HB1,
HR 2) are a case in point. These hybrids were extremely suscep-
tible to downy mildew and ergot. . In a year of severe epidemic, the
whole produce had to be destroyed due to the hazard to consumer
health posed by ergot infected grains. In any case, susceptibility to
a disease would lead to economic losses to the farmers, and often
would lead to the rejection of a variety. In view of these, disease
resistance of a strain is evaluated during each year in which a strain
included in various yield trials.
Quality Tests. Quality tests are conducted to determine the suitabi-
lity of an entry for various uses of its produce. For example, in
case of wheat, the chapati and bread making qualities of grains are
important for their acceptance by consumers. The quality" of a crop
is not a simple character, and is often not easy to determine. There-
fore, a number of different tests have to be conducted for which
sophisticated equipments may be -needed for speedy and reliable
results. In each crop, a different set of quality tests have been deve-
loped in order to determine the suitability of new strains of the crop
450
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Release of New Varieties
respective All India Coordinated Crop Improvement _ Projects. The
purpose of Agronomic Trials is to determine the suitable dates of
sowing and the optimum number of irrigations. The plot size in
wheat Agronomic Trials is 10 x 1.61 m and the number of replica-
tions is 3. In wheat, two types of experiments are conducted :
varieties X dates of sowing, and varieties X number of irrigations.
Three dates of sowing are used with each variety ; early (last week
of October), normal (15-25 November) and late (second week of
December). The three levels of irrigation are : one irrigation, two
irrigations and adequate number of irrigations. The data from these
trials are generally not essential for the consideration of ah entry
for identification, although they are included in the proposals for
varietal identification.
4 . Model Agronomic Experiments . These experiments are conducted
under the All India Coordinated Agronomic Research Project (and
not under the coordinated project for improvement of the concerned
crop). In Model Agronomic Trials, only those entries are included
that have been identified by the workshop of the respective coordi-
nated projects. These trials are conducted at all the centres of the
l Coordinated Agronomic Research Project in the zone.
I National Trials. The national trials are conducted throughout the
country in all the zones. The entries in National Trials consist of
one entry from each zone that ranked first in that zone in URT of
the previous year. The National Trials serve as IET for an entry in
zones other than that in which it was developed and tested in URT.
The purpose of National Trials is to evaluate outstanding entries of
one zone in the other agroclimatic zones to see if they perform well
in other zones as well. It has been suggested that National Trials
should include IET entries of all the zones so as to evaluate all
entries on a national level. This is undoubtedly desirable in the identi-
fication of entries adapted to various zones, but it would increase
the number of entries in the national trials to an unmanageable
level in a crop like wheat.
Adoptive Research Trials. Adoptive Research Trials are conducted on
research stations or farms of state governments. The entries identified
by the workshop of the concerned coordinated project are included
in these trials. The plot size is 0.5 acre (0.2 hectare) and there is no
replication. The agronomic practices used in' Adoptive Research
Trials are the same for which the concerned entry has been identi-
fied. The data from these trials are considered for release of the
identified entry as a new variety. Thus, each identified entry has to
undergo adoptive trials before it is released as a variety.
Minikit Trials. The Minikit Trials are conducted in the farmer’s
fields. These trials are conducted alongwith the Adoptive Trials in
the following crop season when the entry is identified for lelease.
The Minikit Trials are conducted under the supervision of Director,
High Yielding Varieties, Ministry of Agriculture and Irrigation, 5
Government of India. The Minikit Trials are conducted at 300 m
riant meeting : mnctptes ana Metnoas
516 .
for identification in > the ^ i5e?tifie?for° it j?
improvement P r0ject R if Jh Trials, Model Agronomic Trials and
EM
r^in adoptee 1 trials Agro-
Mtrv as a variety. X he oa ta ir & P cc^niial for the release of a
noimc Trials and Minikit j^s * r ; J c \ ria!s are included in the
variety, although those fror n ag ^ ^ evaluated for one year m
release proposal. Thus whea e identi fi ca tion by the workshop
^ "* c “'™' ^
Release Committee. rm ns varies to some
The duration of different tna m mung bean, there is no
extent. In most pulses, except gram * uded * in CVX (Coordi-
IET and URT ; the entries a t e “ ire y , ars . i n Jowar, the ;
nated Varietal Trial) and evaluated y Sorgh um Trial, three ;
evaluation consists of one x ? al f t wo years of Adoptive Trials,
years of Advanced Sorghum Trials and ^ ^ jn lET> one year
In Oilseeds, the en . tn ^ B .£ r fNat j ona i Evaluation Trial) and one year
in CVT, one year in NET (Nat size is three-times the ;
in AT (Adoptive Trials). In NET the plot ^ C VT and one
Sto S&* Inert } entry forprc .
year NET are required for the lacnu
release multiplication. j£T to URT
The criteria for the promotion f ^ secon d/third year I
/ Af PVT) for the retention for evamauon t j> e workshop !
URT/CVT, and for the ide “ t f C j se 0 f wheat, only those entries j
vary from one crop ^ the other^I ^ in URT which belong |
are promoted from. IET to URi or w etop Slgm . J
■ to the top nonsignificaat grm P n tr ics whos0 yields are not significant- 8
ficant group consists of th .• hest yielding entry in the given 1
ly inferior to the y ield . of have an acceptable level of 1
i want In addition, such ^ _ *thp average coefficient ofin-'il
resistance to diseases ( maIn ty*' HiL’der irrigated and less than 20 g
fection (ACI) must be less than 1 : \ le vels of resistance g
under rainfed conditions. Smtlar^ havc been defined.
to important diseases and pests 1 and / or pests may not be an J
But in some crops, resistan retention of entries in trials. F° r |
important criteria for promotion/retention^^ ^ the tri#ls are conjl
Mamnle in the case of rapeseed an chiefly because entries
ducked uSder lea f 1
SS^ids’r g^S^JSStS susceptible
separate evaluation of disease tpo f
is carried out (Tab.e it ip . SUDe rior performance ill
*j R * *SSft£ f tot year t Included' in Agronomic Trials of th j
wool!
morse
Monc
in pi
Mom
and yi
Locate
moose
carry!.
they a
(identi
Ft ger
cfarom
show
would
surviv<
critica;
Chrom
the rec
which
monos
has aM
chrj fl |
,l oted that zones have i
t;Z? I po, :f. lcaI boundaries, but on the basis of
ttons prevailing m the area. Thus, it j
agroclimatic conditions at any two locations within
more comparable than those at two locations!
The various trials conducted under the
particularly in the case of wheat, mav be grou
S? r S^ (, !r £ial eValuat,on trial® (IE
cxpeilmlms ’ <3) agr ° n0mic tria,s ’ and
l. initial Evaluation Trial ( fET ). A ne
initial evaluation trial within the zone
strain included in 1ET or URT is co
ft* . W1,] be used in the above se.
•I he IE t is conducted at 10-12 locati
size in wheat IET is 6 X 3.38 m, and t
generally 6. In wheat, IET is conducte
conditions : (I) timely sown, irrigated •
(J) timely sown, rainfed ; URT is a!s<
three conditions. The conditions under
crops are evaluated are summarised in
the conditions of evaluation vary from
there are live different evaluation coi
strains are tested under nine differenf
mally included in only one of the above i
The entries are tested in IET for one year
%ttZZ R l 8 i 0na J TriaU «/ their
n°t^ been created oo the
— - agroclimatic condi-
may be expected that the
a zone would be
in any two zones,
coordinated projects.
ns witain a zone. The plot
! e number of replications is
i under the following three
J) late sown, irrigated ; and
conducted under the same
. new strains of various
able 27 2. In most crops,
two to four. But io arhar
htions, while in peas the
mndilions. A strain is nor-
iree types of IET of wheat.
inly ; they are promoted to
erjormance is outstanding
ue crops, e.g., pulses and’
r ^ore than one year. An
ted under the same condi-
e objective of IET is to
outstanding entry promoted to URT i« te
tion in which it was evaluated in IET 7
toT’mV 6 reIa 5 i . ve, y inferior entries so’ th<
to a manageable size for URT evaluation
2. Uniform Regional Trials (URT). Th
both in yield and disease resistance in
unde P rT e ° ? l ° ^ ^. niform Regional
lumber ?f «pLSnsffs°Slly
„ , URT , ma y be continued for one
)ased on the performance in URT
asifoVf < hpf anCe f arS re - iec!ed - N ew enti
asis of their performance in IET »f the
‘ n for; one year in URT, the outstir
-meye™ C In r i'h ,S ’ and ^ evaIlla tion h
RT inf 1 he case of wheat > after evr,
RT entries superior in yield and disease
the FT 7 if" 6 outsta nding
the IET of the previous year
Trials The URT is conduS
at 25 T% ned entf y was
iirt 3 -° Afferent locations
URT is 6X2.76 m and the
514
Plant Breeding : Principles and Methods
Zones * Evaluation
procedure**
TTnwHZ, NWPZt CVT S 3 Yrs
NEPZ. EZ,
Mustard
5 (HZ, NWZ,
CZ. WZ, EZ)
Oilseeds
LET. 1 Yr
C.VT, lYr
NET, 1 Yr
AT. lYr
— do—
Yellow sarson
Brown sarson
Groundnut
— do —
6 (NZ. WZ, CZ,
SEZ, SCZ, SZ)
Linseed
Safflower
Soybean
Cotton
4 (NWZ.Indo-
gangetic
alluvium,
CZ, PZ)
2 (Traditional :
Maharashtra,
AP, Tamil Nadu
Non-traditional '•
the rest of the
country)
4(NHZ. NPZ,
CZ. SZ)
3 (NZ, CZ,
SZ)
Conditions of testing
1. Small-seeded, timely sown
2. Bold-seeded, timely sown
(both rainfed ; irrigation
if required)
L Timely sown, rainfed
2. Timely sown, irrigated
L Early maturity, irrigated
2. Late maturity, irrigated
3. Rainfed
1 Timely sown, irrigated
2. Timely sown, rainfed
Rainfed
A. 1 Rabi, irrigated
B. Kharif. rainfed
2. Bunchy
3. S'emisprc ding
4. Spreading
1. Timely sown, irrigated
2. Timely sown, rainfed
1 Rainfed
2. Irrigated; only at Phalfan
(Maharashtra ; 2 irriga-
tions)
Cash Crop
insjyy MP7 IVT, 1 Yr 1 . Irrigated (all zones)
ry cyV ’ AVT 3 Yr 2. Rainfed (AVT and ZET
C ZET* 2 Yrs in CZ and SZ)
i /\i7 07 A . Desi Cotton
on 3(NZ, LA 1. Varieties
. ^ 2. Hybrids (G. harbaceumX
G. arbor euni)
B. American Cotton
3. Varieties • Early maturity
4. Varieties : Late maturity
5. Hybrids ' CZ and SZ o >ly )
“Tttj't vr^rth^rn Hills Zone* NPZ. Northern Plains Zone; NVVPZ, Noith-
NHZ, Northern Hills ZjOnc, Fsstern Plains Zone; EPZ, Eastern
Western Plains Zone; NLPZ^ Nath Extern ^ ^
Plains Zone, FEZ, Far . North-Western Hills Zone;
Zone; SHZ. Southern ’ z Western Zone; SEZ South-Eastern
eS Hz! H In* Zone, NWZ, North-
'SCZ, Sowo-Ccntral Zone; SZ.
Trial; PST, Prehmi nary y j^ a{ j ona j Evaluation Trial (plot
CVT .Coordinated ^uetal ^ oil eeds entries identified for
size three-rimes the » M p Tl . pvt Preliminary Varietal Trial;
size three-nmes the .>iz o ^ Ss^px)* PVT, Preliminary Varietal Trial;
!='«»' I*** AVT. Advanced
varietal trial; ZET, Zonal Elite Trial.
I
h
it
W
450
wouh
mortc
Monc
in pi
Monc
and w
Local,
mouse
carry!
they a
(identi
Ft ger
chrom
show
would
survive
critica.
Chrom
the rec
which
monos
h 2LS • '
raBsH -■■T1 .'4
| ■/ "JO' I
Release of New Varieties
6 (Zoom i, 2,
3* 4, 5,
and 6)
I (the whole
of India)
I (The whole
of India)
SSSS.
Mungb.an
Rajm®
procedure 0 *
Millets
Stage, 0
(Zonal)
Stages 1-3
(All India
basis)
PST, 1 Yr
AST, 3 Yrs
AT, 2 Yrs
UVT» 3 Yrs
AT, 1 Yr
Poise®
CVT, 3 Yrs
7 (NWHfZ, <
NEHZ,
NWPZ, WZ,
nepz, ez,
C Z)
(EXACT
and
ea cr, ?
Zones ;
ACT-i and ACT-2,
o Zones ;
ACT-3, 2
Zones)
9 CNWHZ, U
NWPZ, WZ, c
NEPZ, EZ, a
SEZ, CZ, SZ, n
NEHZj oi
(Late sown, 7 t ,-,
zones ; bold- S c
seeded, 7 zones; JE
Kabuli, 4 zones) 3 '
7 (NWHZ r
NWPZ.EZ,
NEPZ, CZ,
NEHZ, WZ)
(Dwarf trial,
5 zones;
vegetable peas,
single zone)
8 (NW HZ, jrt j v
NE HZ, WZ, CZ, CVT 2 Yrs
nw| nepz, SvT"
(Spring, 7 zones ; CWrT'i'rs •
f~- Summer; ’
1 zone . CVT 3 YrO
^ (NEPZ, CZ) CVT, 3 Yrs
JET, I Yr
CVf, 2 Yrs
AT, I Yr
(In categories
other than
timely
sown, no
JET; CVT,
3 Yrs)
CVT, 3 Yrs /
A. Late maturity (>90 day,)
r M r ? um mat . ur »y (85-90 days!
C. tarty maturity (80-85 da y ri }
D Extra Early (80 days) y ;
L Hybrid sorghum trials
2. Varietal sorghum trial*
I. Bajra varieties trials
2 Bajra hybrid trials
I. Extra early (EXACT
maturity
100130
formal (ACT.,, , 30lJ60
4 d'fys) 0 ® (ACT ' 2> 160 200
\ L A a c'r<t C IXZ%!>
one irrigation at flower-
mg) '
i. Timely sown, irrigated
r sown » irrigated
3. Bold-seeded (>20g
weight), irrigated
(alldesi-types)
4. Kabuli, timely sown,
imgited
A. Field peas (white-seeded,
timely sown. Irrigated)
1 • Normal height
2. Dwarf
B. Vegetable pea®
(green-seeded)
: - 3 * rhy Ceptil ' ie * Chrly matUm
4. Susceptible, medium
5. Susceptible, 2ate
J* Resistant, early
7. Resistant, medisim ■
Resistant; late
I. Kharif, timely sown
irrigated/rainfed
A Spring, timely sown,
irrigated
3. Summer, timely sown
irrigated
Timely sown, irrigated
'Quality Seed : Classes , Production Practices and Maintenance 529
Similarly, only two crops (wheat arid maize) were covered in 1966-
67, while quality seeds . of more than. 70 crops are now being pro*
dueed. The Values quoted in Table 2£.l do not include tire quahtity
of seed produced and marketed by private seed ‘companies
THE INDIAN SEED ACT (1966)
The Indian Seeds Act was enacted in 1966, and has been in.
force since Oct 2, 1969 in all the States and Union Territories of
the Indian Union. This '-a'et aims at regulating the quality of
seed sold for agricultural purposes through compulsory labelling
and voluntary certifeaMbn. Under compulsory labelling , anyone
sdlfihg the 'seed of a nbrifted Msi® br Variety, m the region for which
it has been notified, should ensuife ttet (!) the seed conforms to the
prescribed limits of germte&tfdh and purity, (2) the seed container
is labelled in the prescribed m&mrer, and (3) the label truly
represents thfe quality bftte Vh the contairitr. Under vofantdry
certification , any one -interested in producing certified sded may do
sd by kpplyihg to the seed certification agency For the grant of a
TAiSe 2BX The growth 'of In India as indicated by the quantities
of qudlityAMtffi^d seed produced In some important crops. (Based
pn, R L. ‘Agrawal, 197 S ; and Anonyteou.s 1987. AgHcuftoral
Statistics at a glance. Ministry of Agriculture, Govt, of India).
Crop
Quantity df. quality] certified seed (in lakh quintals)
1966-61
1967-68
1973-74
1980-81
1984-81
Wheat
0,24
Q.B
1.49
WA7
it 09
Rice
—
0.05
mi
■6.41
=8,49
Maize
0.32
<H*3
V.08
0.85
1.51
Jo war
—
—
1.51
■221
Bajra
—
—
—
1.64
1.59
Other cereals
—
—
—
—
6.22
'Gram
—
*-
—
—
0,66
UVd
—
—
■ — ,
■—
0.32
Peas
—
—
—
—
0.23
Other pulses
—
: : — .
—
1. 06*
0,88
Grouncfh'Ot
—
—
—
—
4.67
ka'ple/in'ustard
—
—
(— *
049
Soybean
—
—
0.02
—
0.90 .
Other oilseeds
—
—
—
2.28**
0.47
Cotton
—
—
—
1.19
lute
—
—
0,22
Potato
—
1.69
12.25***
Others
. 7-T ' ;
‘0.602
0.06
—
0.07
rm\
■0.36
: 0.998
2 22
25 01
48 46
♦Total for all the pulses ; crop^Wfs© quantities toot
•♦Total for all- the oilseeds.
e
wotll
jhou(
Mou<
in p,
Mom
and v
Local
moos*
carryi
. they a
(ideat
Fa gei
chwm
show
would,
survive
critica
Chrom
the re<
which
monos
has ab,
chrorgl
in
the prescribed standards and procedures. There are two bodies
riz., The Central Seed Committee and the Central Seed Certification
Board, which advise the central and the state governments in the
matters related to the general administration of the seeds act and
of seed certification, respectively.
CLASSES OF QUALITY SEED
The various classes of seed that are used in a seed production
programme are , (1) breeder seed* (2) foundation seed, (3) rcsisterwT
seed, and (4) certified seed. These classes of seeds were first clearlv
defined by the International Crop Improvement Association in 1946
m relation with fodder and forage crops ; in 1968 it recommended
the adoption of the same system m the case of grain crons wii
These different classes of seeds have different requirements 5 and serve
different functions, a brief description of which is given below.
Breeder Seed Breeder seed is the seed or the vegetative propagating
material produced by the breeder who developed the particuk?
variety. It is produced by the institution where the variety was
developed in case the breeder who developed the variety k nit
available. In India, Breeder seed is also produced by other Agri-
culture Universities under the direct supervision of the breeder of the
concerned crop working in that University; this arrangement is made
in view of the large quantities of the breeder seed required everv
Breeder seed is used to produce the foundation seed. y
In case of self-pollinated
. -.'1 to retain the genetic
are promptly eliminated and
or natural hybridization
is obtained from breeder seed by
genetically pure and is the source
Production of foundation seed is
‘ 1 * s produced on Govern-
. _ ‘ T-Jre Universities or
supervision of experts from
j n area a( j a p ta ^
produced from foundation seed
^ genetically pure and is
seed. Registered seed is
according to technical
Often registered seed is
from foundation
is produced from foundation, registered
L we it is certified by'' a seed
year.
Breeder seed is genetically pure.
species, mass selection is regularly practised
purity of the variety. Oif-type plants
care must be taken to prevent outcrossing
and mechanical mixtures.
Foundation Seed. Foundation seed
direct increase. Foundation seed is g<
of registered and/or certified seed,
the responsibility of NSC. Foundationseed L
meat farms, at experiment stations, by Agricultu
by competent seed growers under strict super
NSC. This class of seed should be produced
tion of the concerned variety.
Registered Seed. Registered <
or from registered seed. Reg
used to produce certified seed.
usually produced by progressive "farmers
advice and supervision provided by NSC.
omitted and certified seed is produced directlv
seed. This is the general practice in India.
Certified Seed. Certified seed i t / A
or certified seed. This is so known because
Quality Seed : Classes. Production Practices and Maintenance 531
certification agency, in this case State Seed Certification Agency, to
be suitable for raising a good crop. The certified seed is annually
produced by progressive farmers according to standard seed produc-
tion practices. To be certified, the seed must meet certain rigid
requirements regarding purity and quality. These standards vary
from one crop to another and shall be discussed later in some detail.
Certified seed is available for genera! distribution to farmers for
commercial crop production. Its production is generally by State
Seeds Corporations, but NSC also undertakes the supervision of
certified seed production, if required.
«. eem Remarts
produced by
Originating breeder
sr experiment station
{/) Genetic purity
rigidly maintain-
ed. Mass selec-
tion often prac-
tised
00 Source of all other
classes of seed
(0 Genetic purity
maintained
(«) Progeny o i breeder
seed
NSC/SSC at their
farms under strict
.control ; at other
stations as well
FOUNDATION
SEED
(/) Genetic parity
maintained
(ii)$ Progeny of fount*
dation or regis-
tered seed
((Hi) Omitted in India.
Progressive farmers
under technical
guidance and super-
vision from SSC
(*) Certified by State
Seed Certification
Agency
(//) Must conform to
rigid purity and
quality require-
ments
(Hi) Used for commer*
cial crop produc-
tion
Progressive farmers
under!? supervision
from SSC
DISTRIBUTED TO FARMERS
FOR COMMERCIAL CROP
PRODUCTION
Fig. 28.1. Steps in seed multiplication.
The various classes of improved seed, are recognised to facilitate
the maintenance of genetic purity of the variety and to ensure a
continuous supply of good quality seed at a reasonable cost. It also
532
Plant Breeding : Principles and Methods
helps in multiplication of the seed rapidly while maintaining the
purity of the seed. The production of breeder and foundation seeds is
very costly since a very high standard of purity must be maintained
The requirements for certified seed are relatively less rigid than
those for foundation seed, hence it is considerably cheaper. The
various steps in seed multiplication are outlined in Fig. 28.1.
Sometimes, the seed produced under unfavourable weathei
conditions may meet the requirements of purity etc. but mav be
smaller in size and poorer in germination. Such seed maV be
classified as substandard seed, and issued tags to that effect* Sub
standard seed has to fulfil the requirements for phvsical and genetic"
purity, and freedom from weed seeds and diseases 'as prescribed for
certified seed. Such a seed may be used when standard certified seed
is not available for growing commercial crops. ~ '
REQUIREMENTS FOR CERTIFIED SEED
Seed has to meet certain rigid requirements before it can he
certified for distribution. The first and foremost requirement is that
the seed must be of an improved variety released by either the Central
or “ S * a l e Y ar ! e jy. Relea / e 'Committee for general cultivation and
notified by the Ministry of Agriculture , Government of India • this h
essential for the seed to be certified. The other requirements are
related to genetic purity, freedom from weeds, diseases and ‘ nests
germination etc. The various requirements for certified seeds are
summarised in Table 28.2 for some of the field and vegetable cron-
It may be noted that there is considerable variation in the reouire'
mepts for certification in various crops. In certain cases el
maize (Z. mays), the requirements are more rigid than in the others
These requirements are briefly described below.
Genetic Purity. Genetic purity means the absence of seeds of other
varieties of the same crop species as well as of other crop species
Genetic purity ensures that the seed is of the variety under certified
rion, and that there is no mixture from other varieties or other crops.
The standard of genetic purity is very high ; the amount of contami-
nation permitted ranges from 0-0.1 percent. In most of th» dies
contamination by seeds of other crop species is permitted to a small
degree (up to 0.1 per cent), but contamination by seeds of other
varieties of the same crop is generally not permitted'ijable 28 2).
Physical Parity Physical purity implies freedom of seed from inert
matter and defective seed. .Inert matter consists of nonliving materials
such as sand pebbles soil particles, straw etc. Defective seeds are
those seeds that are broken, diseased, insect infested, shrivelled and
unfit for germination A broken seed larger than half of the normal
seed is not considered defective provided the embryo is not damaged
The total amount of permissible contamination by inert matter Tnd
defective seed ranges from 1.0 per cent in the case of maize (z Lvst
to 5 per jflt m the case of carrot (Table 28.2). t • mays)
Quality Seed : Classes, Production Practices and Maintenance 533
Table 28.2. Germination and purity standards for certified seeds of different
Held and vegetable crops.
Pure ■ Other
seed crop
(min.) seed
(max.)
Per cent of total seed ( on weight basis)
Other Total Objection - Total Mo
varie- weed
ties seed
(max.) (max.)
Field Crops
able weed
seed
(max.)
inert ture,
matter (max.)
(max.)
Hybrid maize
(other than
single cross)
98
0.2
—
None
—
2
12
90
Maize compo-
sites and open-
pollinated
varieties
95
1.0
—
None
—
2
12
90
Hybrid jowar
and varieties
Hybrid bajfa*
98
0.1
0.1
None
2
12
so
and open-pom-
nated varieties
98
0.1
— .
0.1
—
2
12
75
Rice*
98
0.1
o.i
5 seeds/
40 teg
2
13
SO
Wheat*
98
01
—
0.1
5 seeds/kg
2
12
85
Barley
98
0.1
— -
0.1
5 seeds/kg
Z
12
85
Cotton varieties
and hybrids
98
0.1
—
0.1
- —
2
10
‘60
Gram
98
o.ds
—
None
—
2
9
85
Arhaf
9'8
0.2
—
0.2
—
2
10
•75
Urid
98
0.2
—
05
—
2
9
65
Muflg '
itapeseed and
98
0.2
0.1
*■*—
2
9
75
mustard
91
0.5
— -
0.5
0.1
3
8
85
■Sesamuip (Hi)
91
0.2
—
02
—
3
9
80
Groundnut'
96
None
—
None
—
4
9
70
"Sunflower
98
None
—
None
. —
2
9
60
Linseed
9t
0.2' 4
— .
0.2
—
2
1
so
Soybean
97
0 05
0.5 0.10
Vegetables
3
12 •
.70
Peas
n
0.05
0.5
None
— w
2
9"
' 75
Cow peas
m
0.05
0.2
0.1 •
2
9
75
"Tomato
98
0.1
None
2
%
19
Cauliflower .
98
0.1
02
—
2
7
65
BhihdT
'Watermelon
99
0.05
—
None
None
1
10"
.65
and other
cucurbits
99
0.1
0.2
None
Nope
1
7
60
..Onion ' :
9$
0.1
02
' ■
2
8' '
7o
Carrot
95
0.1
*— ; '
0.2
5
: 8
60
Chillies
98
0.1
0.1
— '
2
8
>60
Radish
98
0.1
—
0.2
—
2
6
70
Brinjal
None
—
None
~
2 '
8
70
♦Diseased seed standard (max.) ; 1. Bajra, ergot sclerotia 0.04% (by nuojtser)
2. Wheat ; nematode gails of tundu ear cockle
or Kama! bum 0.5% (by number)
3. Rice, paddy bunt - 0.5% £by number)
wouI«
monc
Monc '
in pi
Monc
and v
Local
monsc
carry!
they a
(icient:
Fi ger
ChwiT;
show
would
survive
critica:
Chrom
the rec
which
monos /
has abjf
mC r;
Plant Breeding : Principles and Methods
Germination. A high percentage of germination is necessary to obtain
a good crop stand with the minimum amount of seed. The percent-
age of germination required for certification is high in a crop like
maize (90%), moderate in several others, e.g., jowar (80%), wheat
(85%), rice (80%) etc., and relatively low in many others, e.g., cauli-
flower (65%), bhindi (65%), carrot (60%) and chilli (60%). Thus
the minimum standard of germination for seed certification varies
considerably from one crop to the other (Table 28.2).
Freedom from Weed Seeds. Freedom from weed seeds is necessary
to prevent weeds from spreading through seed and to reduce losses
caused by weeds. The maximum amount of weed seeds permitted is
very low ; it varies from zero per cent in crops like maize (Z. mays )
and tomato (L. esculentum ) to 0.2 per cent in cauliflower
(B. oleracea ), onion (A. cepa ), carrot (D. carat a) etc. For some crop
species, some weeds are classified as objectionable or noxious weeds.
In such crops, the certified seed is required to be free from seeds of
the noxious weed species.
Freedom from Diseases. The certified seed must be free from seed-
born diseases. If the seed is contaminated with pathogens, it is
likely to lead to an epidemic and a total loss of the crop. The
presence of seed-born pathogens is prevented in two ways ; first, by
1 effective control of diseases in the standing crops, and second , by
treating seeds with disinfectants and protectants. The maximum
permissible frequency of diseased seeds is given below Table 28.2.
Optimum Moisture Content. The seeds must be dried to an optimum
moisture level (Table 28. 1) for efficient processing and safe storage.
The level of optimum moisture varies from one crop species to
another. It varies from 13 per cent in case of rice to 7 per cent in
the case of cauliflower and watermelon (C. vulgaris ). A moisture
content higher than the recommended level (1) leads to a loss in
seed viability, (2) promotes growth of microorganisms, particularly
moulds, (3) favours attacks by storage insect pests, (4) increases the
chances of damage to seeds by fumigants, and (5) interferes with
processing because damp seeds tend to stick together.
REQUIREMENTS FOR FOUNDATION SEED
The foundation seed, like certified seed, has to fulfil a set of
rigid quality requirements ; these standards are summarised in Table
28.3.^ The standards for pure seed (minimum), inert matter*
(maximum), moisture content (minimum) and germination (minimum)
are comparable for foundation seeds with those for certified seeds of'
various field and -vegetable crops. However, the maximum permis- *
sible limits for the quantities of other crop seed, seeds of other
van ties, total weed seed and seeds of objectionable .weeds are much
lower in the case of foundation seeds than those for certified seeds.
For example, in the case of, jowar the maximum permissible limits
for other crops seed and total weed seed are 5 seeds/kg in the
case of foundation seed, while they are 0.1 % in the case
of certified seed (Tables 28.2 and 28.3). Similarly, in the case of
Quality Seed : Classes , Production Practices and Maintenance 535
Table 28 3. Germination and purity standards for foundation seeds of some
field and vegetable crops.
Pure Other Seeds Total Objection -
seed crops af other weed able weed
(min.) seed varieties seed seed
(max.) (max.) (max.) (max.)
Total Moss - Ger~
inert ture mina -
matter (max.) Hon
(max.) . (min.)
Field Crops
Maize 98.0 0.2, 1.0** — None
Jowar 98.0 5/kg — 5/kg —
Bajra 98.0* 10/kg — 10/kg —
Rice 98.0* 10/kg — 10 kg 2/kg
Wheat 98.0* 10/kg — 10/kg 2/kg
Barley 98.0 10/ kg — 10/ kg 2/ kg
Cotton 98.0 5, 'kg — 5/kg — -
Oram 98.0 None — None —
Arhar 98.0 0.1 — 0.1 —
Urd and
Mimg 98.0 0.1 — 0.05 —
Rapeseed
and
mustard 97.0 0.1 — 0.1 0.05
Til 970 0.1 — 0.1 —
Groundnut 96.0 None — None —
Sunflower 98.0 None — None —
Soybean 97.0 None 0.1 5 seeds/kg — ‘
Vegetables
Peas 98.0 None 0.1 None —
Cowpeas 98.0 None 0,1 None —
R&jma 98.0 None 0,1 None —
Tomato 98.0 0.05 — None —
Cauliflower 98.0 0.05 — 0.05 —
Bhindl 99.0 None — None None
Watermelon
and other
cucurbits 99 0 ' 0,05 ' — None None
Onion 98.0 0.05 - 0.10
Carrot 95.0 0.05 - 0,1
Chillies 98.0 0.05 — 0.05 —
Radish 98.0 0.05 — 0.1 —
Brinjal 98.0 None — None —
12 80,90***
12 80
12 75
13 10
12 85
12 $5
10 65, 60
9 85
10 75
^Diseased seed standard : 1. Bajra ; ergot sclerotia, 0.02% (by number)
2, Wheat ; nematode galls of tundu/ear
cockle or seeds infected by Kama! bunt,
0.10% (by number)
3. Rice ; paddy bunt, 0.10% (by number)
"For hybrid and other varieties, respectively. It includes seeds of other
distinguishable varieties, V; *
For single cross hybrids and other varieties, respectively.
450
Wt> Breeding : Principles and Methods
gram the maximum permissible limit for seeds of other varieties
is 0. land 0.5% for foundation and certified seeds, respeSv
In the case of rapeseed and mustard, the maximum permissihi*
quantity of objectionable weed seed is 0.05 and 0.1% for foundation
and certified seeds, respectively. a
Foundation seed crops is subjected tp field inspections in the
same manner as those for certified seed. However, the standard
for isolation distance (Table 28 4) and those for field inspections
(Tables 28.6 and 28.7) are much more stringent in the case of foun-
dahon seed than those for certified seed. The State Seed Certification
Agency (t>SCA,> carries out the field inspections and seed tests for
foundation seed and. if the standards are met with, it issues the
appropriate tags for the same. CS tfte
OPERATIONS ESSENTIAL TO A SEED INDUSTRY
The following operations/activities are essential to a success,
ful seed industry : (1) breeding of new varieties, (2) seed multiplies
tion, (3) seed processing, (4) seed certification, (5) seed storase’
(6) marketing and distribution, and (7) publicity. 8 ’
Breeding of New Varieties. Breeding of new improved varieties of
various crops is the basis for a seed indnstrv _
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Quality Seed : Classes , Production Practices and Maintenance 537
must be stored in a rpooi with low temperature and relative humi-
lity ; the National Seed Project is developing such facilities for
the storage of breeder seed at different centres in the country.
Generally, processed seed is stored in bags which are arranged
iq suitable lots to facilitate inspection during storage. The height of
(he |3ag lots should not be more than. 2 — 2.5 m for crops like
soybean, groundnut etc. having seeds that are more readily damaged,
and 3-f| m fpr crops like ?ice, wheat, maize, etc. which have relatively
less damage prone seed.
Seed Marketing And Distribution. A proper distribution of seed is
an important operation and presents a major problem in India
since most of the farmers are illiterate. Seed distribution is the
primary responsibility of the state governments and is being done
|hfpugh extension and agricultural development agencies, e.g
development blocks, agriculture officers, officers for specific crops
etc. Otner^ agencies involved" in seed distribution are, private
traders, qgrjculture universities, cooperative societies etc. The health
of* the seed industry of a country depends, to a large extent, pn the
efficiency of its marketing wmg r
fed Certification. Seed certification is the responsibility of the Seed
■Certification Agency (SSCA) of the concerned state. In those states
where a SSCA does not exist, National’ Seeds Corporation (NSC)
discharges this function. SSCA undertakes certification of seeds
on the written requests of seed growers.' For this purpose, SSCA
■carrier out field inspections and seed tests to ensure that the seed
■crop. $nd the seed meet the rigid quality requirements prescribed
for the concerned plass of seed : on being satisfied, it issues appro-
priate tags to the seed growers for affixing them to seed bags. 'The
main objective of seed certification is to keep a rigid control on
the quality of seeds offered for sale by various agencies!’
PaMtcity. Both the farmers and the seed industry stand to gain
from the popularisation of quality, seeds of improved varieties,
Seeds of new varieties may be publicised by inserting suitable
advertisements in news papers, agricultural, magazines, radio, T.V.
etc.’ National Demonstration trials, minrkit trials, Kisan melas ,
Kisan goshthis etc. serve as extremely, powerful tools for the popular-
isation of new varieties and the benefits -from their quality seed.
In fact, the most farmers in the country need no more to be educated
on the importance of quality seed ; they need to be provided with
only the information on which varieties . to use, and where to get
their quality seed. It is more urgent to ensure that the simple
farmers are not cheated in the name of quality seed.
.'SEED PRODUCTION AND PROCESSING
Production of high quality seed requires considerable technical
skill and a number of rigid requirements must be fulfilled in order
to ensure a high purity and germination of seed., Seed multipli-
cation involves, two- separate steps" : (1) 'seed production, and '
450
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438
Flam Breeding : Principles and Method*
(2) seed processing. Seed production requires improved cultural
practices, efficient weed, disease and pest control, optimum irrigation
and fertilizer inputs and some other specific operations. Therefore
only progressive farmers are given the responsibility for seed produc*
tion since they have all the required inputs for raising a good seed
crop. Seed processing is ordinarily done by State Seeds Corporations
who have the facilities for large scale processing of seed But a
farmer may develop his own facilities for seed processing if he k
producing seeds on a large scale. e -
Seed Production
Seed production, for convenience, may be dealt with in twn
subheads : (1) isolation, and (2) cultivation. m two
Isolation. The crop raised for seed production should be separated
from other fields of the same crop species by a minimum distant
which vanes from one crop to the other. This distance is known
as isolation distance. Isolation is essential to prevent pollination
from unwanted pollen in the case of cross-pollinated and ofceifetdk*.
pollinated species and to avoid mechanical mixture
cross-poilmation m self-pollinated speciesi The ssdtetwn dteribe
Table 28 4. Minimum isolation distance for seed ceniScattoh Jasoine crons.
Minimum
isolation
Crop distance
Itofattondkstantit
md
seed
" p&mm ■
Hybrid maize
Hybrid jo war
(a) other jowar
(b) Johnson grass
Hybrid bajra
Wheat
400
300
400
1000
3
200
200
400
200
3
Yes
No
No
No
Yes-
No
No
No
'dates
Yu
Yes
Y m
Ym
Cotton
Soybean
Rapeseed .and
mustard
Groundnut
Sesame
Peas
Cowpeas, sem,
rajma
Tomato
B hindi
Chillies
Potato
Brinjal
Carrot
Cucurbits
Radish and
turnip
Cauliflower
3
50
3
400
3
100
20 '
50
50
400
400
200
2,000
ZOO
3
30
3
200
’ 3
50
10
10
20
200
100
100
100
400-800
400
1,600 1,000
1,600 1,000
Quality Seed : Classes , Production Practices and Maintenance 539'
varies from 3 m in self-pollinated crops like wheat, rice, etc. to
200 m in the case of maize, bajra and jo war to even 400 m in case of
jowar when the. isolation is from Johnson grass (Sorghum halepense)
(Table 28.4), In some cases, e g., hybrid maize, the minimum isola-
tion distance may be considerably reduced by planting border rows
of the pollinator parent and by choosing a larger held for seed
.production. But in certain other crops, e.g., jowar and bajra, such a
modification is not permitted. Even in case of maize, the isolation
distance cannot be modified if the seed colour of the seed parent is
different from that of the contaminating maize, or if the contamina-
ting field is planted with sweet corn or popcorn. In any case, it is
necessary to plant a variable number of border rows of the male
parent in case of hybrid seed production to ensure a sufficient supply
of pollen on the edges of the field.
Table 28.5. Modification of the minimum isolation distance in hybrid maiw
. (Z. mays) seed production by the area of the seed field and by
border rows of the pollinator (male) parent.
Area (ha) of the seed field
Minimum number
of b&rder rows of
the male parent
4
8
12
16
Isolation distance (m) from other maize
200
190
m
170
1
175
165
155
145
3
150
140
130
120
5
87.5
77.5
67.5
57.5
10
50
40
30
20
13
It may be pointed out that the minimum isolation distance
required for a foundation seed crop is markedly greater than that
for a certified seed crop in the case of all those crops where pollen
contamination is feared ; only in the case of strict seif-pafmators
like, wheat, rice, soybean etc., the isolation requirements for the
two classes of seeds are identical, e.g. t 3 in. In most cases, the
isolation needed for foundation seed crop is twice as much, or even
more, as that needed for certified seed crop.
Cultivation. Improved cultural practices and recommended levels of
fertilizer and irrigation must be provided to raise a healthy crop*
The objective is to harvest the maximum yield of a high quality
'seed. The following important points should be observed for raising
a good seed crop.
Land Requirement The land should fee level, fertile and free from
noxious weeds common to the crop. In some cases, like bajra and.
jowar, the field to fee used for seed production should not have been
used for growing the same crop in the previous year. If the same
crop was grown in the previous year, the field should be irrigated J
weeks before' sowing and ploughed just before sowing. This is ' done
to allow the seeds that might have dropped in the field from the v pre-
' vious; crop to 1 germinate,- and to destroy the seedlingsfby ploughing. :
450
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540
Plant Breeding : Principles and Methods
Cultural Practices. Recommended improved cultural practices
roust be followed for raising a good 'seed crop. Recommended does
of fertilizers' and irrigation water must be applied for high yields' of
high quality seed.' Poor cultural practices 'Would give lower yields
and seeds of smaller size which would be rejected at the time of
grading. This would ' drastically reduce the profits of seed
growers.
Plant Protection. Adequate measures must be taken to protect the
seed crop from diseases and pests. Insect pests and diseases ' may
cause considerable damage to the crop' reducing the yield and
quality of seed. Further, incidence' of' some diseases may lead to
rejection of the seed by the certifying agency as unfit for use as seed
For example, the maximum permitted incidence of head smut and
grain smut in jowar (S. bicolor, seed pqrent only) is one fieacfin
10,000 and one head in 2,500, respectively, at the time of last
held inspection. This rigid requirement with respect to diseases is
prescribed to prevent the occurrence of disease epidemics due to
contaminated seed.
Ifeed control. Effective weed control is a must for good seed produc-
ts?. Weeds reduce crop yields and weed seeds contaminate'fhe
seed. Certain weeds are classified as objectionable weeds by "the seed
certification agency and the seed field is generally required to be free
from sSch weeds. For example, in case oT hy brid bajra, no objection-
able weeds should be present at any field inspection ; if tbev are
present, the seed field may be rejected. " -
Roguihg. Roguing is the removal of plants which are off- type, that is
-%!‘S n0t Xl.? a y different from the plants of the variety under cer'tifica-
tldn. ft is aa important aspect of seed production and is necessary
to prevent outcrossing and mechanical mixture. The off-type «ianis
are regularly removed from the field either by uprooting orby
2 < he 3round level. The off-type plants may differ fn pldS
heighf, leaf characters, Sowering time, maturity etc: P
Special Operations. Seed productiop in certain crops may reauire
SS° P r £iq ^' F Of example' hybrid jnaize seed pd25
Fnrth^ d - e i Se |! n -l ° f l he femal< r parent before they shed pollen
Farther m hybrid seed production, 'the rows of male parent are
Srn §f ^Q he r0WS ° f fet0ale ' parcnt t0 ?void mec hdniba!
and . thre * hin « °f a -'eed crop should be
Zu om?dera ^ le care m °**der to prevent mechanical mixture
clZ aid L C r° P K? ed ° r weedseeds The threshing boor should be
2^, r ?i Pr?fetaWy cemented to keep the contamination by inert
ravo.fdamSrseeds^ “ * *** dur ^ ^ing so'as
Seti Stacessing
, w The seed obtained from the threshing floor is contan
have exbllT’nfh^ r Cro P ? e . ed and weed Seed, is pf different
e excess moisture and its germinabrfity is not known.
Quality Seed : Classes, Production Practices and Maintenance 54 1
is processed, tested, treated and bagged before it is offered for sale.
The various steps in seed processing are, (1) drying, P) cleaning and
grading, (3) testing, ( 4) treating, (5) bagging and labelling.
Drying. Seeds must be dried to an appropriate moisture level (Table
28,1) in order to facilitate processing, to prevent losses in germina-
tion and to reduce the chances of insect attacks during storage.
Drying generally involves the removal of moisture distributed inside
the" whole seed. This moisture first reaches the surface by capillary
action from where it evaporates. Thus drying of seeds is a relatively
slow process. Drying of seeds from rabi crops generally presents no
problems since the weather at the time of harvesting is hot and dry.
But in the case of kfaarif crops, and in some years in the case ’of rab
crops as well, seed drying may become a major problem due tv
unfavourable weather conditions at the time of harvest.
Seeds may be dried (1) naturally or (2) artificially.. Natural
drying is done by spreading the seeds on hays, floors or fields in the
open sun. Air movement and heat generated by the sun rays dry the
seeds provided weather conditions are favourable. In case of un-
favourable weather conditions, drying must be done artificially.
Artificial drying involves passing of heated or unheated air through
the seeds to remove moisture.
In unheated drying, normal atmospheric air is 'passed through
the seed. For effective drying, the air should have low relative humi-
dity ; therefore, it is not effective in a moist weather. It is a slow
process and a larger drying space is needed. Slow drying may lead
to the growth of moulds and thereby cause damage to seeds. It has
the advantage of requiring low initial cost, no expense on fuel and
there is ho danger of fire. ‘
Heated air method involves passing of heated air through the
seed. This method is quicker, faster end requires less drying space
than the unheated air method. Drying is not affected by weather
conditions as is the case with the unheated air method. The dis-
advantages of this method are : high initial equipment cost cost of
fuel, danger of fire and possibility of overheating the seed which may
reduce germination.
Cleaning And Grading. The seed from threshing floor' is mixed with
seeds of other crops and of weeds, pieces of straw, gravel, soil
etc. Further, the seed is not of uniform size, but contains seeds of
several sizes some of which are undersized, shrivelled and unfit for
use as seed. Separation of inert matter , weed seed and seeds of other
crops from the seed is known as cleaning. Seeds from different crops
differ in size, shape, weight, specific gravity, surface smoothness,
colour, electrical properties, stickiness etc. Machines are available
that separate seeds on the basis of one or the other of the charac-
teristics listed above. For example, specific gravity separators divide
seeds on the basis of their weight and size ; pneumatic separators
542
Plant Breeding I Principles and Methods
separate seeds on the basis of their resistance to air-flow, spiral
separators separate them on the basis of seed shape ; velvet-mil
separators on the basis of surface smoothness ; electronic separators
on the basis of electrical properties of seeds and electronic colour-
separators on the basis of seed colour.
Grading is the removal of smaller and shrivelled seeds from the
- well filled healthy seeds. In India, air and screen machine is exten-
sively used for cleaning and grading of seed. This machine uses air
•current for separating seeds on the basis of their resistance to air
stream and uses sieves to separate seeds on the basis of their size
and shape. Commonly, the air and screen machine has either two or
three screens ; the size of the screens varies depending upon the
crop. Dried seed is passed through the air .and screen machine for
cleaning and grading.
Testing. After cleaning and grading, the seed lots are tested for
percentage of pure seed, weed seeds, seeds of . other crops, inert
matter and germination. This is known as seed testing and is done
in a seed testing laboratory. Seed testing is an integral part of every
seed certification programme and is used as a check on the quality of
the seed to be marketed. The seed certification agency carries out
seed tests on the seed lots presented for certification. We shall
discuss in some detail the process of seed testing a little later.
Treating. Before bagging the seeds are treated with a suitable
fungicide, often in combination with an Insecticide. Seed treatment
is helpful in the following ways : (1) it is helpful in controlling seed-
born diseases, such as bunt in wheat, grain smut in jowar. seedling
blight in maize, rice, jowar and wheat, Fusarium wilt of jowar and
wheat etc., (2) it protects seeds from seed and seedling rots caused
by Phythium and Rhizoctonia commonly present in soil, (3) protects
against damage by storage pests, and (4) protects from’ damage by
•soil insects io the field.
There are two classes of seed treatment chemicals : disinfect-
ants and protectants. Disinfectants inactivate the organisms present
on the seed surface, e.g ., organo-mercurial compounds, like Agroson
GN, Cereson etc. Protectants protect seeds from the attack' by seed-
born and soil-born pathogens, e.g Thiram, Captan, Chloranii
etc. Organomercurials are injurious to seeds, particularly to cracked
seeds. Therefore, they are used when disease incidence in the seed
crop necessitates it and in crops where seed is not easily injured,
e .g. 9 in rice, wheat, barley, cotton etc. Organomercurials are not
used in such crops as maize, bean (P. vulgaris ). peanut ( A . hypogaea)
and vegetable seeds. An appropriate insecticide is often combined
with a fungicide to protect the seed from Insect damage. The
chemicals may be applied as dry powder or in the form of slurry ;
the latter provides a more uniform coverage and is commonly used,
ihe recommended schedule for seed treatment of some crops are
summarised in Table 28.6. F
Quality Seed : Classes, Production Practices and Maintenance 543
Table 28.6. A summary of the recommended schedules for seed treatment.
Chemical and its
formulation
(fungicide)
Nat a re of
treatment
Quantity per 100 kg seed
Chemical Water in case of
(g) slurry (/)
Cereals And Mil!*
Thiram 75 % WDP S
or Captan 75% WDP S
EMC or MEMO; or W
Organo-mercurial 1 % D
Thiram 75% WDP S
or Thiram 75% dust D
or Captan 75% dust D
Organo-mcrourial l %* D
Brine** (salt solution)
Thiram 75% WDP S
or Captafol S I
Thiram 75% WDP S
or Maneozeb S
Organo-mercurial 1 % D
Carboxfn*** S
or Carbendazim*** D
■Gram Captafol
Urd Thiram 75% dust
or Carbendazim
50% WP
Gowpea Captan 75% WDP
or Thiram 75% dust
Mung Thiram 75% WDP
or Captafol
Carbendzaim
Arhar Thiram 75% WDP
Pulses
S
Groundnut Captan 75% dust
or Thiram 75% WDP
Captafol
Rapeseed and PM A 1% dust.
mustard or Thiram 75% dust
Soybean * Captan 75% dust-f
Thiram 75% dust
or Maneozeb
or Captafol dust
S
Oilseeds
D
Sufficient to completely im-
merse the seeds
Solution sufficient to com-
pletely immerse seeds
Captan 75% dust
or Thiram 75% WDP
EMC, MEMO
250 —
ioo 0,5
Immerse the seeds for 6 firs,
in a 0,2% solution ,
Plant Breeding : Principles and Methods
Vegetables
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In cm®
Bhindi Thiram 75% WDP
'or Captan 75% dust
Bnnjal Thiram 75% dust
or Cap tan 75% dust
Carrot Thiram 75% dust
Cauliflower
etc, Thiram 75% dust
Chillies Thiram 75% dust
or Captan 75% dust
Cucurbits Thiram 75% dust
or Captoo 75% dust
Onion and
tomato Thiram 75% dust
Peas Captan 75% WDP
dr Thiram 75% WDf
Note*. I. For all seeds, add DDT 50% WP to the Weighed quantity of fungi-
cide at the rate of 100 g DDT per kg of the fungicide, mix them well
and treat the seed with this mixture.
2. D» S 'and W denote dry dressing, slurry dressing, and wet treatment,
respectively.
3. Where options are given, the chemicals (fungicides) are listed in
the order of preference.
4. EMC, ethyl mercury chloride ; MEMC, methoxy ethyl mercury
chloride ; PM A, pheByl mercury acetate.
5. ^denotes that the seeds may be treated only if the grain smut
infection in the held was more than the standard prescribed for
certification.
6. **denotes that seed p?ay be treated only if ergot infection in the
field was present within the prescribed limits for certification ; fields
having more infection than the limit are rejected.
7. Seed must not remain in brine solution for more than 5—10
minutes. After removal of ergot sclerotia, the seed must be
repeatedly washed with clean water to remove all salt from the seeds,
8. ** ^denotes a specific treatment against loose smut. Only seed
meant for raising a seed cfbp may be treated.
Bagging and Labelling. After seed treatment, seeds are distri-
buted in bags of appropriate size (generally, 40 kg bags are used) ;
the process is known as bagging . Each bag is labelled with an
appropriate label (Fig. 28.2) which carries the following information :
(1) kind of seed, (2) name of the variety, (3) purity, (4) per cent
germination (5) date of germination test, (6) per cent weed seed,
(7) per cent inert matter, (8) name and address of the seller!
(9) ; period of validity of the certification, and (10) anv other infor-
mation pertinent to the seed.
Accurate labelling is important to the purchaser as it provides
the necessary details about the seed. Seed laws require that accurate
information is provided on the label The Indian Seeds Act of i966
is designed to regulate the quality of seed offerred for sale.
quality Seed : Classes , Production Practices and Maintenance 545
J’lnwwf,
'Gerajnation not below tTSV
Pure Seed 99%
Lot No J
Other Crop Seed 0%
Inert Matter not, more then
’Price--
Weed Seeds 0%
Certification valid upfco. _ .
Producer*
450
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Plant Breeding : Principles and Methods
through seed. There is a specified maximum for each of the above
that is permissible during’ any field inspection ; the limit varies to
Table 287. The maximum permissible frequencies (as per cent of plants) of
off type objectionable weed plants and plants infected by desig-
nated diseases in certified seed crop at any field inspection.
Crop
ojf-'-yp*
plants
in separable
other* crop
^ plants C%),
Qrjctionahle
weed plants
Plants
infected by
inf desig-
nated
diseases
Pollen
producing
plants of
the seed
parent
Hybrid maize
(other than
single cross) 0 5
—
—
— '
1.0
Composite/
open-pollinated
maize 1>0
—
—
—
Hybrid jowar 0.05*
. —
—
0 I*
0.1
Jowar vane
.
ties
0.05
—
—
0.1
——
Hybrid bajra 0.05*
—
0.1**, 0.04**
0.1
Bajra OP
varieties
1.0
—
—
0.1**, 0.04**
— ■ ,
Rice )
Wheat X
0.3
0.05
0.02
0.5
—
Barley )
Urd
0.2
—
—
’ —
, ;
Arhar and
gram
0.2
Moong
0.2
—
0.2
Hybrid
Cotton
02
T
,
~~ .
Rajma
02
—
—
0.2
Pea
0.5
— ~ ..
Cowpea
0.2
—
—
0.2
— ' .
Rapeseed and
mustard
0.5
0.1
0.2
Til
0.2
—
—
J.O
—
Groundnut
0.5
—
—
—
Soybean
0.2
—
—
—
Bhindi
0.2
None
0.2
—
Cauliflower
etc.
0.5
—
0*5
—
Tomato
0.5
—
0.6
—
Brinjal
02
—
—
05
—
Carrot
0.2
—
—
•—
Bhiili
0.2
—
—
0.5
—
Cucurbits
02
—
—
1.0+
—
Onion
0.5
—
—
—
— '
i
I
?
\
♦Both in the pollen and seed parents
**ln the seed parent alone ; g reen- ear 4- green smut and ergot, respectively
♦♦♦In the swd parent alone, erect disease
h Only in muskaielon ; virus diseases.-
Quality Seed : Classes, Production Practices and Maintenance 547
some extent in different crops (Table 28.7). In case of cross- and
often cross-pollinated crops, a very rigid check is applied on the
presence of off-type plants during the various stages of flowering in
order to prevent contamination through pollination. In each crop, a
specific number of field inspections must be made at the specified
stages of crop growth (Table 28.8). For example, in case of hybrid
bajraseed production, three field inspections must be made at the
following stages : (1) before flowering, (2) during flowering, pre-
ferably at or near full flowering, and (3) before harvest but after the
seed is mature enough to reveal the seed characteristics.
Table 28.8. The minimum number oT field inspections essential for seed
cei tlfkation in some crops
Crop Minimum number Stages of crop growth at which field
of field inspection inspection is done
Hybrid maize, jowar 4
and bajra
Other varieties of. 2
maize
Jowar varieties 3
Wheat, rice, barley, 2
gram, arhar, pea, mung,
urd, groundnut, soy-
bean, cotton, cowpea,
rajma
Rapeseed, mustard, til, 3
pea, cucurbits-, tomato.
brinjai, bhindi, chillies.
Carrot 3
Onion (transplanted) 4
Potato 3
One inspection before flowering, two
inspections during flowering, one ins-
pection before or during harvesting.
One inspection before flowering and
one during flowering.
First inspection before flowering,
second during flowering and third
before or during harvesting.
Anytime from the initiation of
flowering till maturity.
First inspection before flowering,
second during flowering and third
after fruits .mature; before harvesting.
First, 20-30 days after sowing, se-
cond, at the time of transplanting
of roots ; and third, during flower-
in?.
First, early stages of growth ; second,
digging out of bulbs ; third, trans-
planting of bulbs; and fourth, dur-
ing flowering.
First, 45 days after planting ; second,
before killing of aerial shoots ;
third, after killing of aerial shoots.
Foundation seed crops are also subjected to the same number
of field inspections as those for certified seed. However, the
maximum permissible limits for definite off-type plants, doubtful
off-type plants, objectionable weed plants, plants infected by
specified diseases and pollen producing plants of the seed parent
(in the case of hybrid varieties) in the case of foundation seed crops
of various crop species are either one-half or less than those for
their certified seed crops (Table 28.9). For example, the maximum
permissible limits, in per cent, for plants of objectionable weeds in
wool*
monc
Monc
in pi
Monc
and vi
Locah
mouse
carry!;
theyj
34 g Plant Breeding : Principles and Methods
rice are 0 0! and 0.02, for plants infected by specified, diseases are
0.1 and 0.5, and for definite off-type plants are 0.05 and 0.30 for
foundation and certified seed crops, respectively.
Table 22.9. The maximum permissible frequencies (% of plants) of, off-type
plants, objectionable weed plaats, an.' plants infected bydesig-
sxated diseases in a foundation seed crop at aoy field inspection.
Objectionable Plants Pollen
weed infected producing
plants (%> bydesig- plants of the
nated seed parent
diseases (%}
(%)
Definite
off-type
plants
<%)
Sigls cross
hybrid maize
0.1 (both
the
parents)
Composite and
opeo-poliinated
maize
Hybrid jowar
iowar varieties
Hybrid bajra
tfajra O.P.
varieties
Puce )
Wheat t
Barley )
Moo ng
Urd
Hybrid cotton
Gram
Araar
Cowpeas
Rajma
Pea
Rapeseed
&nct mustard
Groundnut
Soybean
Bbindi
Cauliflower etc
Tomato
Briojal
Carrot
Chilli
Cucurbits :
Onion
None
*Green tar + green smut aod ergot, respectively,
**On!y in musk melon ; virus diseases.
Quality Seed : Classes, Production Practices ana Maintenance 549
Table 27,10. A list of objectionable weeds and designated diseases for seed
production of some crop species.
Toria and
mustard
Cowpea
Moong
Cucurbits
Objective weeds
Convolvulus arvensis
(Hindi, hirankhuri)
Nil
Argemone mexicana
(Hindi, satyanashi)
Wild cucurbits
Wild Abelmoschus
species
Nil
Designated diseases
Loose smut ( UstHago tritici )
(Pers ) Rostr
Grain smut ( Sphaceloiheca
sorghi (LK.) Clinton)
Head smut i Sphaceiotheca
reihana) (Kueho) Clinton)
Grain smut (Tolyposporium
penicillariae Bref )
Green ear ( Sckrospora
graminicola (Sacch) Schr.)
Ergot ( Claviceps microcephala
(Wall.) Tul.)
Alternaria blight' (Alternaria
brassicae (Berk.) Sacc.)
Anthracnose ( Colletotrichum
copsicoe (Syd) But!. & Bisby
Ascochyta blight Ascochyta
phase ■ lorum S:icc.)
Bacterial Blight ( Xanthomonas
compestris pv. vignicola)
Halo blight ( Pseudomonas
syringae pv. phaseoh )
Pea mosaic ( Pisum virus 2A; Syn.
pea virus 3)
Pumpkin mosaic [Cucums
virus)
Yellow mosaic ( Hibiscus Virus 1)
Brown rot {Pseudomonas
solancicearum E.F. Smith)
Leaf roil (potato virus 1 ; Syn.,
Solanum virus 14).
Mild mosaic (potato vims X ;
Syn., Solanum virus 1),
Root Knot nematode
{Mehidogyne sp )
Golden nematode of potato
( Globodera rostochiensis Wr)*
Scab ( Streptomyces scabies
(Tbaxt) Wakoman & JHemrid)
Severe mosaic (potato virus Y)
Wart (Synchytrium endobioticum
(Schilb) Perc ) .
*)t occurs in the Nil-gin Hills area ; movement of seed potato grown
in this area to other areas of the country is strictly prohibited.
in case of some crops, certain weeds are classified as noxious
or objectionable weeds (Table 28.10) because their seeds are similar
to and very difficult to separate from those of the concerned crops,
550 Plant Breeding : Principles and Methods
they are difficult to eradicate once they infest a held and/or they
may act as hosts for diseases or insect pests of the respective ctopl
Similarly, certain diseases of some crops are designated (Table
28. JO) -since their pathogen is transmitted through seed (propagule)
Objectionable weed plants and plants affected by designated diseases
are specifically monitored during field inspections.
Inspection During Seed Processing. The inspection during seed
processing is done to determine if the seeds have been dried to the
appropriate moisture level, and if the correct processing procedure
is being followed. In case of maize, a check is also made to see
that undesirable cobs have been separated and rejected before
shelling. Another purpose of such inspections is to determine if
appropriate care is taken to avoid mechanical mixtures during seed
processing. ’
Seed Tests
Seed tests consist of a series of tests designed to determine the
quality of seed. Seed tests are done in seed testing laboratories
Almost every state has a seed testing laboratory which performs the
following functions.
1. Conducting research on seed testing methods.
2. Training of personnel in seed testing.
3. Determining the standards for seed purity and seed quality for
various crops.
4. Seed testing for certification and for implementation of seed
laws of the country. The latter implies that the seed tests
conducted by the seed testing laboratories are accepied as
evidence by the courts of law in India.
Before certification, the seed lots are subject to seed tests in
seed testing laboratories. The seed has to meet a specified standard
before it can be certified. The minimum standards for seed certi-
fication vary to some extent from one crop to the other (Table 28 1)
Generally, the following tests are conducted to . -determine' the
quality of seeds : (1) purity test, (2) germination or seed viability
test, and (3) moisture content test.
Sampling. Seed tests are conducted on small samples drawn gene-
rally from processed seed lots. It is essential that the samples used
for seed tests are a representative of the lot. In other words the
woul*
mcnc
Monc
in pj
Monc
and vi
Local,
mouse
carry!;
they a
(iderMjg
Quality Seed : Classes , Production Practices and Maintenance 55 i
two bags of the seed lot. The composited seed is thoroughly mixed
and is subdivided into a number of smaller samples, known as
working samples . The weight of a working sample is usually 25 g.
Often the seed samples are mixed and subdivided into working
samples with the help of certain machines, e.g^ Bcerner Divider,
Garnet Precision Divider.
Parity Test. Purity denotes the percentage of seeds (by weight)
belonging to the variety under certification. The working sample is
closely examined, often with the help of a magnifying glass, to
classify it into the following components.
L Pure seeds ; seeds of the variety under certification
2. Seeds of other varieties of the same crop
3. Seeds of other crops
4. Seeds of weeds/objectionable weeds
5. Inert matter ; sand, straw, stones, pebbles, soil particles etc.
6. Defective seeds ; broken and shrunken seeds. A broken seed
which is larger than half of the original size and has intact
embryo is classified as pure seed. The defective seeds are
classified as inert matter.
The components 2 to 6 are impurities ; there is a maximum
permissible limit for each of these impurities in the seeds of different
crops (Table 28.1). The purity of the seed is calculated on the weight
basis as follows.
Weight of pure seed
Purity (as per cent)=
X100
Similarly, Impurity (%)=
Total weight of the working sample
Wei ght o f pure^seed _ y 100 •
Weights of (pure seed 4- seeds
of other varieties 4- seeds of
other crops + weed seeds +
inert matter)
Weights of (seeds of other
varieties+seeds of other crops
•T weed seed + inert matter) x ^
Total weight of the sample
Impurity percentage is also referred to as dockage. The purity test is
done on two or more samples from the same seed lot to make sure
that the results are resonabiy reliable.
Separation of seeds of other varieties of the same crop from
those of the variety under certification is often more difficult than
that of other impurities, e.g. f other crop seed, weed seed etc.
In many crops, e.g.. arhar, gram, rajma etc., seeds of different
varieties may differ in colour, shape, size and/or other morpho-
logical characteristics ; this permits a relatively easy separation
of other varieties seeds. In case of wheat, phenol reaction of
seeds is used as a basis for determining the frequency of seeds of
woul
mom
Monc
in pi
Morn
and w
Local,
monst
carryi;
they a
552 Plant Breeding : Principles and Methods
other wheat varieties. For this test, seeds are presoaked for 24 hr
and placed on filter papers soaked in a 1 % phenol solution (usually
kept in petri plates). Usually, a 4 hr period is allowed for the
seeds to develop colour ; sometimes a longer period may be allowed
to permit a more accurate identification. Seeds of different varieties
differ in their reaction to phenol, and this provides "the basis for
their identification. This technique may be used in barley also.
In some crops, e.g., Aneva, Lolium, Pisum etc., seeds of other
varieties are identified with the help of ultra violet (UV) light
Seedlings of many crops, e.g., Triticum, Gylcine, Sorghum, maize
sugarbeet, moong etc., develop anthocyanin, and their varieties
may differ for this characteristic. In such cases, seeds are germinated
generally in petri plates, and the seedlings are classified on the basis
e of their anthocyanin pigmentation (colour). Some special treatments
I may be used to intensify the anthocyanin pigmentation of seedlings
e.g., germinating wheat seeds on filter papers soaked in 1 % solution
of sodium chloride, and exposing the seedlings to UV light for 1-2
hr before observation. Other seedling characteristics, e.g , hairiness,
intensity of chlorophyll pigmentation etc. may also be used for
identification of seeds of other varieties.
A grow out test may be resorted to for a more definite deter-
mination of seed purity. In such va test, at least 100 plants are
grown with proper spacing and under optimum management either
in a green house or in an off-season nursery. Various morpho-
logical featues of the plants, including flower colour and other
floral features, plant height, days to flower, days to maturity, ' fruit
and ear head features etc. are scrutinized in an effort to determine
the frequency of other varieties seeds in a seed sample. A grow-out
test is perhaps the most precise but is rarely used in this country as
it requires a much longer time, an excellent greenhouse/off-season
nursery facility, a much greater effort and funds than the tests
based on seed and seedling characteristics. The test based on seed
morphology is the most rapid and quite simple and the least
expensive, but its applicability is quite limited. Anv test for purity
must be rapid, inexpensive and simple since a large number of seed
samples have to be screneed for purity.
Seed Viability or Germination Test. Germination is determined as
per cent of seeds that produce or are likely to produce seedlings
under a suitable environment. Thus germination test is of great
importance because the sole function of seed is to produce healthy
se2 j f° r raising a good crop. The two tests most commonly
used for the determination of seed viability are germination test and
tetrazohum chloride test.
Gemination Test. Germination test determines the percentage of
seeds that produce healthy root and shoot. In most of the cases,
seeds are germinated on wet filter papers placed in Petri dishes.
I he Petri dishes are kept under controlled conditions in an incubator
or m a culture room. For most species, a temperature between
Quality Seed : Classes, Production Practices and Maintenance 553
1 8-22°C is adequate ; for some species a specific temperature may
be required. Generally, diffuse light or even darkness suffices.
Where required, specific environment should be provided for germi-
nation tests. The duration of germination test varies from 7-28
days depending upon the crop species. For most of the cereals and
many legumes, 7 days are enough, but in many grasses longer
periods are required. Germinated seeds are counted at regular
intervals and are removed from the Petri dishes. The total number
of germinated seeds would be the sum of the number of seeds that
germinated at different observations. The per cent germination is
calculated as follows.
Germination (%)
Total number of seeds germinated
Total number of seeds plated
XlOO
For convenience, 100 seeds are plated in each sample. From each
seed lot, 4 or more samples are plated for a reliable germination
estimate. If there is a difference of 10 per cent or more in the
germination of different samples from the same seed lot, it is
desirable to repeat the germination test.
Tetrazolium Method. This method determines the percentage of viable,
seeds which may be expected to germinate. The chemical 2, 3, 5-
tetrazolium chloride, or tetrazolium chloride in short, is colourless,
but it develops intense red colour when it is reduced by living cells.
This phenomenon is used to determine the percentage of viable
seeds in a seed sample. Seeds are soaked in tap water overnight
and are split longitudinally with the help of a scalpel so that a
portion of embryo is attached with each half of the seed. One
half of each seed is a- placed in a Petri dish and covered with 1%
aqueous solution of tetrazolium chloride for 4 hours. The seeds are
then washed in tap water and the number of seeds in which the
embryo is stained red is determined. The per cent of viable seeds is
computed as follows.
, .... Numbe r of half seeds stained red w .
Viable seeds ( /£)— Total number of half seeds
The tetrazolium method is faster than the germination method
and it does not require a controlled environment which is necessary
for the germination test. It is relatively cheaper than germination
test. But it cannot be applied to all the species, particularly to those
species that have very small seeds and embryos, because splitting
and examination of such seeds is tedius.
Real Value of Seed. The real value of seed is the percentage of a
seed sample that would prod, ce seedlings of the variety under certifica-
tion. This is also known as utility percentage of the seed, and is
a function of the purity and germination percentages of the seed
sample. The real value of a seed lot is determined as follows.
Real value of seed (%)
Purity (%)x Germination (%)
100
450
woiil
mom
Alone
in p;
Mom
and u
Local,
®ons<
carryi:
they a
(ident
til
Moisture content (%)= — - 2 x 100
5ac
:i si
554 Plant Breeding : Principles and Methods
Clearly, when two or more seed lots are being compared, their
purity and germination should be taken into account. This is'easilv
done by determining the reai value of seed lots.
Moisture Content. Moisture contei t is determined ax per cent water
content of the seeds Optimum moisture content reduces the deterio-
ration during storage, prevents attack by moulds and insects and"
facilitates processing. The moisture content is determined by drying
the seeds in an oven or with the help of a moisture metre. In the
case of oven method, weighed seed samples are dried at 13Q°C for 90
minutes in an oven. The dried seeds are weighed again. The loss
in weight represents the weight of water lost due to drying. The
moisture content is estimated as follows.
Wj-Wa
W,
where Wj is the weight of the seed sample before drying and W« is
the weight of the seed sample after drying.
Moisture metres measure the resistance of seeds to an elec-
trical current ; the electrical resistance of seeds varies with the
moisture content. Use of moisture meters requires calibration and
a certain degree of technical skill on the part of the user. But
moisture meters are very efficient and extremely rapid ; a large
number of samples can be handled in a relatively short period. b
MAINTENANCE OF IMPROVED SEED
The improved seed sold to the farmers is certified seed Certified
seed may be the progeny of certified, registered or foundation
seed. Ibese seed classes are ultimately derived from breeder ^eed
which is maintained by the breeder or the institution that produced
the vanety. Every attempt is made to keep the breeder seed gcnetic-
ally pure arid to retain the original genetic composition of the
variety. The improved varieties are maintained as breeder seed • the
practices for the maintenance of breeder seed vary from one cron
species to the other mainly depending on the mode of reproduction
of the crop and the nature of the variety to be maintained
Self-Pollinated Crops. Isolation (3 m) is provided to prevent out-
crossing and care is taken to avoid mechanical mixtures. Off-tvnes
r ; i r ° n,ptly f - r0gUed °,™ Every year ’ mass selection is done to
maintain genetic purity (Chapter 10). Several hundred spikes are
selected and separate progeny rows are grown. Off-tvpe progenies
are eliminated ; tbe remaining progenies are bulked. This maintains
, 'JZTJ™? 0f ‘i e , vanec j es : The seed obtained in “
is known as nucleus or basic seed ; it is the source of breeder seed. *
Inbred Lines. Hybrid or synthetic varieties are maintained as inbreds
used m producing them. The inbreds are maintained bv strict hand
pSty a ofinbreds tyPe plaEtS ^ pr01T!pi,y ro S ued to maintain genetic
Quality Seed : Classes, Production Practices and Maintenance 555
AsexuaUy Propagated Crops. Varieties of asexually propagated crops
are maintained by asexual reproduction. Mechanical mixtures are
avoided and off-type plants are promptly eliminated.
Every effort is made to maintain the genetic purity of varieties.
Since the breeder is directly responsible for the maintenance and
multiplication of the breeder seed, he is able to maintain the genetic
purity of the varieties.
SEED PRODUCTION ORGANISATIONS
There are two types of government/public sector organisations'
responsible for seed production and certification in India. The
first type of organisation is represented by the National Seeds
Corporation (NSC) which has responsibilities for the entire
country. The second type of organisations are State Seeds
Corporations (SSCs) and State Seed Certification Agencies (SSCAs)
that have statewise responsibilities. In addition, a number of private
seed companies are also engaged in seed production activities,
e.g Hindustan Lever Ltd., Bombay ; Pioneer Seed Company Ltd.,
Hyderabad ; Maharashtra Hybrid Seed Company Ltd., Jalna ;
Indo- American Hybrid Seed. Bangalore etc. These companies deal
exclusively with hybrid seed production of various crops, e.g.,
cotton, maize, jowar, arhar etc.
National Seeds Corporation
The National Seeds Corporation (NSC) was initiated in 1961
under the Indian Council of Agricultural Research. Later, on 7
March 1963, it was registered as a limited company in the public
sector.' The NSC was established to serve two main objectives :
first , to promote the development of a seed Industry in India
and second, to produce and supply the foundation seeds of various
crops.
Before the establishment of NSC, there was no organised seed
industry in India for any of the farm crops. The seeds were sold by
private traders, often without any check on their genetic purity,
physical purity or germination. With the development of hybrid
maize, the necessity of an organisation for seed production was
urgently felt. NSC was established to fulfil this need, and the
production of hybrid maize seed was its first responsibility. Later
on, it was given the responsibility of seed production of hybrid
jowar, hybrid bajra, wheat, jute, vegetatively propagated crops,
forage species, various vegetables ard other crops. Some of the
functions of NSC have now been delegated to the other two agencies;
State Seeds Corporation and State Seed Certification Agency. NSC
handles foundation and certified seeds of nearly 230 varieties of
70 crops. The present functions of NSC may be summarised as
follows.
1. Production and supply of foundation seed.
2. • To maintain improved seed stocks of improved varieties.
woul
xnonc
Monc
in p;
Mom
and vs
Local,
mouse
carry i;
they a
556 Plant Breeding : Principles and Methods
3. Interstate marketing of all classes of seed,.
4. Export and import of seed.
5. Production of certified seed where required.
6. Planning the production of breeder seed in consultation with
ICAR.
7. Providing technical assistance to seeds corporations and private
agencies.
8. Coordinating certified seed production of several State Seeds
Corporations.
9. Conducting biennial surveys of seed demand.
10. Coordinating market research and sales promotion efforts.
11. Providing training facilities for the staff participating in seed
industry development.
1 2. Providing cer tification services to states lacking established and
independent seed certification agencies.
State Seeds Corporations
The State Seeds Corporations are chiefly concerned with the
production and supply of certified seed, and within the state
marketing of certified seed. State Seeds Corporations have been
recently established in order to reduce the workload of NSC These
corporations were established in view of the great success of and the
impact made by the Tarai Development Corporation (TDC)
Pantnagar (established on February 27, 1969), which had gained a
virtual strangle hold on the seed market of U.P, almost to the
exclusion of NSC. It is honed that the State Seeds C^rpoVitiSs
would be able to 1 unction more efficiently and would be able to
stimulate a faster growth of the seed industry.
State Seed Certification Agency
J The State Seed Certification Agency (SSCA) is resoonsible for
seed certification in the concerned state. The SSCA makes field
inspections and conducts seed tests required for seed certification.
The SSCA performs the following functions : (!) it screens the
applications from seed growers for seed certification and decides on
their fitness, (2) it also checks and verifies the appropriateness of
the source seed used for growing the seed crop under certification
(3) it carries out the requisite field inspections, (4) it conducts
the seed tests, (5) it certifies the seeds found suitable and issues
the appropriate tags both for certified and foundation seeds, (6) it
guides the seed growers on production, processing and distribution
of seeds. (7) it conducts short courses on seed production etc. for
seed growers, , 8) and it participates in other activities conducive to
the development of seed industry, eg., preparing and publishing lists
of plant breeders, seed growers etc.
Quality Seed ; Classes , Production Practices and Maintenance 557
Each state has a State Seed Certification Board which super-
vises the activities of SSCA. Persons involved jo seed processing
and distribution, including businessmen dealing with production,
processing and marketing of seeds, and scientists from agricultural
universities, are the members of this board. In addition, there is a
Central Seed Certification Board which advises the state govern-
ments and their SSCA on the matters of seed certification. The
chairman of the board is nominated by the central government.
The members of the board are drawn from among the officials of
the state departments of agriculture, scientists from the agriculture
universities, and persons from the seed industry. The board may
also appoint committees for specific tasks.
CERTIFIED SEED PRODUCTION IN SOME CROPS
Seed production practices vary to a great extent from one crop
to another. The precautions necessary during seed production are
largely affected by the natural mode of pollination in the crop
species. In self-pollinated crops, fewer precautions are required than
in cross-pollinated crops and in often cross-pollinated crops. We
shall briefly examine the seed production practices in some impor-
tant ci op species, viz., self-pollinated crops, hybrid maize, hybrid
jowar, hybrid bajra and potato. For certified seed production , only
the varieties released by the Central or State Variety Release Com-
mittee and duly notified by the Ministry of Agriculture and Coopera-
tion , Government of India , can be grown .
Self-Pollinated Crops
Seed production in self-pollinated crops, such as rice, wheat,
barley, linseed* most of the pulses and certain oilseeds, is fairly easy
since there is very little outcrossing (less than 5%). The chief
concern in seed production of these crops is to prevent mechanical
mixtures. This is done by isolating the seed crop from other crops
by a distance of 3 m, and by roguing the off-type plants from the
seed crop. The cultivation of seed crop is essentially similar to that
of the commercial crop. The various' operations in seed production
are briefly outlined below.
Land Requirements. Generally, there are no specific land require-
ments. The land should be suitable for the cultivation of the crop
in question.
Isolation. Isolation of 3 m must be provided on each side of the seed
field to prevent mechanical mixture and chance outcrossing.
Cultural Practices. For each crop, a standard package of cultural
practices is available. For raising a good crop, the package of prac-
tices should be strictly followed. If the package of practices is not
followed, there may be a reduction in yield and in seed size. Both,
these would lead *o a lower income to the seed growers.
Plant Protection Recommended plant protection practices must be
followed to protect the crop against the diseases and insect pests.
450
wool .
mom
Mon<
in p;
Mooc
and vi
Lacat
moose
carryi
they a
558 Plant Breeding : Principles and Methods
Seed*born diseases like loose smut of wheat must be controlled
because the presence of diseased plants may lead to the rejection of
seed crop.
Weeding. The seed crop is required to be free from weeds, parti-
cularly from' objectionable weeds. The presence of objectionable
weeds would lead to the rejection of seed crop. *
Roguing. Off-type plants are promptly rogued and so are the plants
of other crops. This is essential to avoid mechanical mixtures and
chance outcrossing. There is a maximum permissible limit for the
presence of off-type plants during any field inspection (Table 28.7).
Diseased plants are removed and destroyed.
Hybrid Maize (Z. mays)
The seed producers are supplied with seeds of two single
crosses ; one of them is to be used as female or seed parent and is
tagged with a yellow label, and the other is to be used as the male
or pollinator parent and is tagged with a red label The female
parent is supplied at the rate of 45,000 seeds per hectare and the
male parent is supplied at the rate of 1 5,000 seeds per hectare This
is the foundation seed and is supplied at the rate of thousand 'viable
kernels and not on the weight basis.
Isolation. The isolation distance is 200 m ; it Can be modified bv
planting border rows of the pollinator parent (Table 28 S) But
when the contaminating maize is sweet corn or popcorn or has
distinct seed colour from that of the seed parent, the isolation
distance cannot be modified. •Different planting dates may be use 1
to modify isolation distance provided there are no receptive silks in
the seed parent at the time of pollen shed in the contaminating
field Theiso!atlon dlst3nce 1S also modified by the area of seed
Cultural Practices. A standard package of cultural practices
available which must oe strictly followed for raisin* a ' hpo h.' s
seed cron RsCGfii mended doses of fertile- - . s ' j U ’ h ea Ithy
- . --tr^VMimcnaea aoses ot rert<‘: iC r must be applied and
mterculture operations must be done to remove weeds eta P
Planting. The recommended planting ratio is 2 pollinator rows to
every 6 rows of the female parent. The male rows should be marked
at both the ends with flag, peg or marker plants like sunnhemp etc.
At least one border row of the pollinator parent should be provided*
on each side of the field to ensure pollination at the edges of the
bclieeJplan P ts. Clng Sh ° Uld ^ 75 ' 9 ° Cm betweeo rows and 20-25 cm
Plant Protection. The maize crop has to be protected against insect
pests, such as, stem borers, hairy caterpillars, army worms grass
hoppers, maize beetle and white ants Plant protection measures are
available for the control of these pests. measures are
Detasseiling Removal of tassel, the male inflorescence, before it has
shed pollen is known as detasseiling. If the female parent is male
Quality Seed ; Classes , Production Practices and Maintenance 559
fertile, detasselling must be done 10 ensure that there is no self-
pollination in the seed parent.- ■ Detasselling is done by holding the
plant with left hand a little below the tassel, grasping the full tassel
in the right hand and removing the tassel by a steady upward pull.
The tassel is then thrown on the ground. In most cases, the tassel
is removed one or two days after the tassel is first visible, but in
some cases, it may be necessary to remove the tassels before they
emerge. Care must be taken to remove the entire tassel. Detasse-
lling is done regularly every day. as long as tassels emerge on the
female plants. Three field inspections would be made during the
pollination period to determine the percentage of female plants that
shed pollen and a very rigid standard is maintained in this regard
(Table 28.4).
Roguing. Off- type plants are promptly removed before they shec'
pollen. There is a rigid requirement with respect to the presence o t
off-type plants during field inspections (Table 28.7).
Harvesting. Male rows are harvested before the female rows and the
produce is removed from the field in order to avoid mixture with the
harvest from the female lines.
Drying. The seeds at the time of harvesting have 30-32 % moisture.
Before shelling, the cobs are dried in drying bins to reduce the seed
moisture to 12 per cent. The grains are shelled, cleaned and graded
before treating and bagging.
Hybrid Jowar (S. bicolor jt
The seed producer is supplied with foundation seed of the,
female parent (7.5 kg per hectare) tagged with yellow label, and of
ttie male parent (5 kg per hectare) tagged with red label. The female
parent is generally male- sterile,
Lacd Requirement. If jowar was grown in the previous season, the
field should be iirigated at least 3 weeks before sowing and ploughed
just before sowing to kill the germinating seeds present in the field.
The field should not have pi an rs of Johnson grass.
Isolation. The minimum isolation distance from grain or forage
jowar is 200 m, and from Johnson grass ( S ’ halepense) it is 400 m.
This isolatioa distance cannot be modified by planting of additional
border rows, but differential planting dates may be used to modify
the isolation distance provided the »dj acoiit field has do pollen
shedding plants at the time the female parent is receptive.
Cultural Practices. The recommended package of cultural practices,
fertilizer application, irrigation, interculture etc. should be followed
to raise a good crop. ;
Planting. Two rows of the male parent should be planted to every
four rows of the ferrule parent, ‘Each male row must be marked with
flags, tags, pegs or marker plants. Four border rows of the pollinator
parent must be planted all round the field to facilitate pollination at
the end of the rows. Row to row distance is kept 75 cm, while plant
to plant spacing is 7.5-10 cm.
Plant Breeding : Principles and Methods
Plant Protection, The crop most be p.otected from shoot fly, stem
borers, ■ spider mites, midge, ear bug and caterpillars. Control
measures are available for effective control of these pests.
Roguiog. Off-type plants must be removed immediately, upon dis-
covery. The field should be kept free of weeds and no objectionable
weed should be present in the field. A rigid standard has to be met
for certification (Table 28.7). Pollen shedding female plants must be
promptly eliminated.
Field Inspection. There would be three field inspections as follows.
t First inspection before flowering to determine isolation, presence
of off type plants, planting ratios and errors in planting,
2. Second inspection during flowering to check pollinating rogues
and male fertile female plants.
3. Third inspection prior to harvest and after the seed has
matured to reveal its true characteristics.
Harvesting. Male rows are harvested before the female rows in order
to avoid mechanical mixture.
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Drying, The seeds may need artificial drying. The moisture content
of the seeds should be reduced to 12 percent.
The seed producer is supplied with 2.5 kg of foundation seed
of the female parent (tagged with yellow label), generally male
sterile, and 1.2 o kg seed of the male parent (tagged with red label).
Land Requirement If bajra was grown in the previous season, the
field must be irrigated at least 3 weeks before sowing and ploughed
just before planting. This is done to induce the seeds present in the
soil to germinate and then to kill them by ploughing.
Isolation. The minimum isolation distance is 200 m. This distance
cannot be modified by planting additional border rows, but it can be
modif ed by differential planting dates. However, the contamination
field should not shed pollen when the female parent has receptive
stigmas. F
Planting. The recommended ratio of male rows to female rows is 2
to 4 The male rows are marked for quick identification At least 8
border rows of the male parent must be planted all round the ‘field to
facilitate pollination Row to row distance is kept 60-70 cm and
plant to plant distance is maintained at 20-22 cm.
Th ^ r f tended package of cultural practi-
k f ° T rai - S ’ ng - a heaIthy seed cr °P- Recom-
mended doses of fertilizer, irrigation and interculture should be
provided
SS -r bee.™
Hybrid Bajra (P. americamum)
Quality Seed.: Classes, Production Practices and Maintenance 561
f
Roguing. Off-type plants must be removed immediately when they
are discovered. Roguing should be done before, during and after
flowering. Pollen shedding female plants are eliminated promptly,
Roguing is done at maturity to remove plants with distinct
characteristics. '‘Roguing in bajra is done either by uprooting the
plants or by cutting them at the ground level to prevent regrowth. A
rigid standard has to be maintained for seed certification (Table
28,7).
Field Inspection. Three field inspections are made as follows.
1. First inspection is done before flowering to determine isolation,
presence of off-type plants and planting ratios.
2. Second inspection is done during flowering to determine pollen
shedding female plants and off-type plants.
3. Third inspection is made prior to harvest when the seed is
mature enough to reveal its true characteristics.
Harvesting. Male rows must be harvested before the female rows to
avoid mechanical mixture.
Drying. Artificial drying may be necessary to reduce the seed mois-
ture to 12 per cent.
Potato ( S . tuberosum)
The chief objective of seed production in potato is to produce
seeds free from viruses Y, A, X and S. These viruses are transmitted
by aphids. Aphid population is low in hills during April to August®
while it is low in the plains during October to early January. The
aphid population in the plains starts building up in the middle of
January and continues till April end.
Nucleus seed. Nucleus seed of potato is produced in the hills during
April to August when the aphid population is low. This seed is
brought to the plains and is stored in cold-storage for planting the
seed crop in October.
Seed Production in Plains. In plains, the main potato crop is taken
during November to March. But in some areasy Jullundur
(Punjab), two crops of potato are taken ; the first crop, is taken from
October to February, and the second crop is taken from January to
May. The potato seed is multiplied in these areas during October, to
mid January. The potato seed is multiplied at Fagu, Jullundur*
Daurala and KodaikanaJ. The seed production practices in potato
are briefly outlined below. . -
1. The crop is planted in the first week of October*
2. The field is not fertilized heavily ; the seeds are planted closely
so that a large number of small, seed-sized tubers are pro-
duced. Roguing of diseased and off-type plants is done very
rigidly.
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Plant Breeding : Principles and Methods
3. By middle of December, irrigation is restricted and it is gradu-
ally stopped by the end of December so that the plants dry up
by the middle of January. The plants may be killed by herbi-
cide spray in the second week of January if they are still green.
This is done to prevent the transmission of viruses to tubers
from the aerial parts due to aphid attack.
4. The tubers are allowed to remain in the soil up to the end of
February or early March. If the land is required for planting
another* crop, tubers may be removed and spread thinly in a
dark place. This is necessary to allow thickening of the skin
which is essential for the storage of tubers in a healthy state,
5. The tubers are harvested, graded and kept in cold storage to
serve as seed for the next year’s commercial crop. The seed for
the main crop and for the second crop in the plains are
obtained from the seed crop of the previous year. One hectare
of seed crop produces enough seed for 10-15 hectares of
commercial crop.
The system of seed production in potato is known as the Seed
Plot Technique. The seed plot technique essentially consists of multi-
plication in the plains of virus-free seed obtained from hills. This is
achieved by growing the seed crop in the plains during the aphid free
period between October to mid January . This seed is virus-free and
serves as seed for the commercial crop of plains in the next year.
SUMMARY
Seed is the seed or any other propagating material used for raising a
commercial crop. The improved seed has four classes : (1) breeder seed, (2)
foundation seed, (3) registered seed, and (4) certified seed. The seed produced
by the breeder who developed the variety, or by the institution where the
variety was developed is the breeder seed . Foundation seed is the progeny of the
breeder seed and is used to produce registered seed or certified seed. Certified
seed is grown by various agencies and is certified for use as seed by the State
Seed Certification Agency. The requirements of good seed are ; (!) genetic
purity, (2) physical purity, (3) good germination, (4) freedom from weed seeds,
(5) freedom from diseases, and (6) an optimum moisture level. The minimum
standards for certification vary to some extent from one crop to the other.
■ Production of certified seed is a rigid process. Recommended standards
of isolation, land requirements, cultural practices, weed control, plant protec-
tion measures, roguing and harvesting must be met for approval of the crop
as a seed crop. After harvest, the seed is processed. Seed processing involves
(1) drying, (2) cleaning and grading, (3) testing, (4) treating, and (5) bagging
and labelling. The seed must be dried to recommended level by natural drying,
or by passing heated or room temperature air through the *^ds. The seed
is then cleaned mechanically to remove weed seeds, seeds of other a op' 5 and
inert matter. At the same time, it is graded on the basis of seed size. The
seed is then tested for purity and viability, and is treated with a suitable fungi-
cide, and often with an insecticide. The treated seed is generally bagged in
40 kg bags and is suitably labelled.
See' 3 Certification is done by State Seed Certification Agency on the basis
of field inspection and seed tests. The ■ seed test is done in a seed testing
Quality Seed : Classes, Production Practices and Maintenance 563
laboratory and consists of* the following nni a,, iati , Anc , / » \ . .
test, and (3) moisture content test ' * 0) purity test, (2) viability
by weight that belongs ,o t“e undS' ^ertlfcation W ? e L CeDt .°f seed
on germination or a chemical test, such as the tetrazoHunfrhWw W P > '. 1S «, , - ased
seeds are those that germinate, or’ mav te expected ^S ^rmin^e ThfP
of seed is computed by multiplying the oer cent p' rT4 of
germination percentage and is expressed as percentage. ‘Moisture continVif
■the seed is estimated either by drying in an oven or by moisture metres?
... Maintenance of improved seed is an exacting orocess Variety nf
an 1 J‘hl t ff^ r ° PS aie purci !" es an , d are maintained by roguing off-type plants
and care , i !° av 9 ld mechanical mixtures. In case of cross-pollinated
^fn°l t , e !r CIi ? SS i' PO i inate u crops ’ inbreds are maintained by hand pollination ■
open-pollinated and synthetic varieties are maintained by onenmohin??;™
isolation. Asexually propagated crops are maintained as clones and off-tvoes
prompt™ eChaniCa 1X1 xturs ’ «*«<» reproduction or mutation arf elided
h vbri/m? ;^ Pr h? '?"• practi u e ? .5 e c°mmended for self-pollinated crops,
defalk ’ h>b ' 3 ° War ’ hybr,d bajra and potato are described in some
QUESTIONS
i. Define the following : seed, dockage, real value of seed, physical purity
genetic parity, roguing,
2 ‘ Yfptf sho ^ v . otes on following ; (/) registered seed, (//) breeder seed
fm) foundation seed, (n>) certified seed, (v) seediest, (vi) germination
test field inspection, (viii) seed certification, (ix) NSC, Lx) SBC
(xi) isolation, (xii) labelling, (xiii) seed plot' technique,
3. Discuss the requirements for seed certification with the help of suitable
examples.
4*; Briefly explain the various classes of seed. Describe in brief the nroc*-
. dare of certified seed production of hybrid bajra,
5. Define seed certification.. Last the _ various operations in certified seed
production. Describe seed processing in some detail.
6. Briefly describe the practices of cultivation of crops for certified seed
production with the help of suitable examples.
7. What is seed test ? Briefly describe the various seed tests for seed certi-
fication. .Discuss the value of seed test in a seed certification pro-
gramme. ,
8. Briefly describe the organisations for seed production in India. Explain
the procedure of certified seed production in self-pollinated crops.
Explain the importance of the following ; grading, drying, seed treatment,
labelling, disease and pest control in a seed crop, germination test,
genetic purity, isolation, sampling in seed test, seed certification, roguing,
detasselling.
Suggested Further Reading
Agrawal, P.K. and Dadlani, M. (eds.X 1987. Techniques in Seed Science
and Technology, South Asian Publishers,' New- Delhi.
Agrawal, R. 1975. Beej Utpadan Evam Pramanikaran. G.B. Pant University
of Agriculture and Technology, Pantna;:ar.
450
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Plant Breeding : Principles and Methods?
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Allard, R.W. 1960. Principles of Plant Breeding. John Wiley and Sons Inc
New York. 9 #1
Chalam, G.V. and Neelkantan, JL 1962. Improved Seed -Agricultural Prn
diaction Manual. ICAR, New Delhi. ai
Gbala^ G.V.. Singh, A. and Douglas, J.F. 1967. Seed Testing Manual.
ICAR and USAID, New Delhi.
Hartman, H.T. and Kester, D.E. 1962. Plant Propagation, Principles and
Practices. Prefritice-Hall of India Pvt. Ltd., New Delhi.
ICAR 1970. New Vistas in Crop Yields— Agriculture Yearbook. ICAR New
Delhi * w
Poehlman* J.M. and Borthakur* D.N. 1969. Breeding Asian Field Crons with
Special Reference to Crops of India. Oxford and IBH Publishing Co
New Delhi. 6
Singh, A. 1963. What is a good quality seed ? Indian Farm. 13 ;
USBA 1961. Seed, Yearbook of Agriculture. Washington, D.C., U.S. Govern-
meat Printing Office. *
CHAPTER 29
vr
Organisations for Crop Improvement
in India
The organisation of agricultural research in India has under-
gone many changes.' It began with the creation of a Department of
Revenue, Agriculture and Commerce in 1871 (chief function collec-
tion of statistics ; no research activity) and has developed into the
present chain of research institutes and agriculture universities with
a scientific manpower comparable to, if not larger than, any single
country in the world. The early development of agricultural research
was associated with the recurrence of famines ; a cruel reminder of
the low priority accorded to agricultural research and development
during the British period. The development of agricultural research
gained some momentum after the first and the second world wars.
After India became independent, much emphasis was laid on agri-
cultural research which has brought rich dividends in terms of
increased agricultural production and near self-sufficiency of the
nation in agricultural commodities. Before we examine the present
organisations for crop improvement, it would be relevant to consider
the history of agricultural research and development in India.
HISTORY OF AGRICULTURAL RESEARCH IN INDIA
Establishment of Agriculture Departments and Agriculture Colleges
The beginnings of a primordial department of agriculture in
In la are as recent as April 27, 1871, when a Department of Revenue
Agncuhure and Commerce was established. The chief function
° i. ' 2 1 c * e P a |' tment remained revenue. There was no work on agri-
cultural development, and the sole function of the department in this
area was the collection of statistics. But this did mafk a beginning
and a recognition, howsoever insignificant, of the agricultural sector.
i y t j ,e xf 0Vern ? e ? t ‘ Tfae credit for J his humble beginning goes to
Lord Mayo the fourth Viceroy of India, and to A.O. Hume a civi-
N a tinnS e r5 enSa ClV ! Servi f e ar3d one ofthe founders of the Indian
^ ; Congress. Ironically, the department was established by
Her Majesty s Government with a view to supply cotton to the
raTished lndia 1 ndustnes of Manch ester, and not to feed the famine
a severe famine during 1877-1878. Based on the
^ h £? mine , commission, the government of India resolved
lo set up a Central Department of Agriculture controlled by the
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Plant Breeding : Principles and Methods
Imperial Secretariat. Simultaneously, agriculture departments were
to be set up in the provinces to look after agricultural enquiry, agri-
cultural development and famine relief. , These departments were set
up in 1881, and directors were subsequently appointed in most of
the provinces. But the chief duty of the agriculture departments in
the centre as well as in the provinces remained famine relief.
In 1892 , an Agricultural Chemist and an Assistant Chemist
were appointed to look after research and teaching. This marked
the first scientific staff in the Department 'of Revenue and Agricult-
ure, as the department was then known. In 1901, an inspector
general of agriculture was appointed to advise the Imperial and the
Provincial Governments on agricultural matters. An Imperial Myco-
logist was appointed in the same year, and an • Entomologist was
appointed m 1903. During the years 1899-1900, India faced the
most severe famine on record. lord Curzon, the then Viceroy of
India, was convinced that the Government of India must pay an
urgent attention to agriculture. As a consequence, an Agricultural
Research Institute (now IARI, New Delhi) was established in 1905
in Push, Hihar (then in the Province of Bengal). The agriculture
departments in the provinces were expanded The provinces were
subdivided into a suitable number of ‘circles’ on the basis of regio-
nal differences in soil and climate. Each ‘circle’ had a research farm*
apd was placed in charge of a Deputy Director of Agriculture.
%tw$en I9QI and 1905, Agricultural Colleges were established or
reorganised at Pune, Kanpur, Sabour, Nagpur, Lyallper (now in
Pakistan) and Coimbatore. It was visualised that the staff at
colleges would combine teaching ;and research to the best ad
tage. But due- to the lack of scientific and technical manpower tad
finance, the primary function of these colleges remained teaching
and training, -
bSIrtwnt of The Imperial Council of Agricultural Keiearel
(The Present-Day ICAR)
As a result of a constitutional reform in 1919, agricultural
development was^mad© a state subject ; centre retained the centra
agencies and institutes of agricultural research and training. Thu?
there was no agency to coordinate the activities of the central insti
tmmm and those of the departments of agriculture in the provinces
Tfe was emphasised by the Royal Commission on Agriculture
appointed in 1926, and headed by Lord Linlithgow. The commission
proposed that an Imperial Council of Agricultural Research should
m set tip to promote, guide and coordinate agricultural research
throughout India. The Council was to act as g clearing house
rtseardt schemes and information, to provide research scholt
and to guide the research activities of the central and pr<
departments of agriculture.
.The proposal of the Royal Commission on Agriculture
examined and the Government of India, Department of Education,
m<B and Lands resolved on 23 May, 1929 to set up an Imperial
Organisations for Crop Improvement in India 567
Council of Agricultural Research. The first president of ICAR was
Khan Bahadur Sir Mohammed Habibullah ; Diwan Bahadur
Sir Vijaya Raghhavacharya was its first Vice-President and
Mr. S. A.Hydari was the first Secretary. The governing body of the
council had 16 members.
The name of the council was changed from Imperial Council
of Agricultural Research to Indian Council of Agricultural Research
in March 1946. The decision to make this change was taken by
the Governing body of ICAR in a meeting presided over by
Sir Jogendra Singh.
The Commodity Committees
In addition to ICAR, there were several Central Commodity
Committees that were concerned with research and development
activities related to specific crops. These committees were started by
the Ministry of Food and Agriculture and were semi-autonomous
bodies. They w^re financed partly by the government and partly by
the taxes collected on the export of the concerned commodities.
Some commodity committees had their own research stations or
institutes located in the main growing regions of the crops concerned
(Table 29.1), while some others financed research schemes conducted
by the State Departments of Agriculture, e.g.. Spices and Cashpwnut
Committee. The Indian Central Cotton Committee was the first
commodity committee established in 1921 on the recommendation
of the Indian Cotton Committee (1917-18). The functions of thi»
Central Committee were cotton improvement, development of im-
proved methods of growing, manufacturing and marketing of cotton.
The committee financed schemes on cotton breeding, diseases, pests,
physiology, agronomy etc. The committee was responsible for the
development of 70 improved varieties of cotton, and the fiber quality
of Indian cotton was considerably improved. In view of the success
achieved in research on cotton under the Indian Central Cotton
Committee, Commodity Committees were set up on other crops,
viz,, lac, jute, sugarcane, tobacco, coconut, oilseeds, spices and
enshewnut and arecanut (Table 29.1). As a result of this, the func-
tions of ICAR became limited to food crops, tuber crops, grasses
and fodder crops, horticultural crops, problems common to all the
crops for which commodity committees were set up, e.g., diseases,
pests etc., dry farming and animal research.
The Vice-President of ICAR was the President of all the Com-
modity Committees. Apart from this link between various commo-
dity committees, there was no other coordination in their activities.
Further, the recearch stations for the various crops were located in
the region where the crop was the most widely grown. But the
soil and climate vary to a great deal from one region of the country
to another, and there was a great necessity to conduct the researches
oh various crops within the different agroclimatic regions of the
country. These realisations led to the formulation of the Project for
450
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Plant Breeding : Principles and Methods
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Table 29.1. The Indian Central Commodity Committees
Name of the Indian
Central Commodity
Committee
Year estab-
lished
Research institute’ station
Cotton Committee
1921
Technological Laboratory (now CTRL)
Lac Cess Committee
1931
Indian Lac Research Institute,
Namkum, Ranchi (1936)
Jute Committee
1936
Jute Agricultural Research Institute,
Barrack pore
Jute Technological Research Labora-
tories, Calcutta
Sugarcane Committee
1944
Sugarcane Breeding Institute, Coimba-
tore
Indian Institute of Sugarcane Research*
Lucknow
Coconut Committee
1945
Central Coconut Research Station,
Kanyagulam and Kasaragod s
Tobacco Committee
1945
Central Tobacco Research Institute,
Rajahmundry (8 substations)
Oilseeds Committee
1947
Financed research schemes ; headquar-
ters at Hyderabad
Arecanut Committee
Spices and Cashewnut
1949
Arecanut Research Station, Vittai
(Karnataka)
Committee
1958
Financed research schemes
tD DDOA n f • , 6 , ivcocan.il on motion, unseeas ana Millets
(PIRRCOm), which was the first step in the country towards coor-
dinated approach to agricultural research.
The Central Commodity Committees were later abolished
S«f,?S R ,h ‘ reSe ‘ rClli0S,itU " S “"' Je ' th ' ir »"«>'
I3tt , 3™cOM)• f,,W, "* l KeSearC " “ Oil***,
A need was felt to coordinate the research on various crons
f'.frV COtt T«'° 1Se - eCjS an< ?- mi ! lets > and aiso t0 conduct the research
work in different agroclimatic regions of the country. The first
coordinated research work on regional basis was initiated in 1956 as
a joint effort by ICAR and the Indian Central Committees on
Oilseeds and Cotton. Seventeen centres were established throughout
aro unZ ri fl Tab e 29 ,' 2) t0 conduct ^search on cotton, castor,
groundnut, Brassica, ul, tona, taramira, jowar and bajra These
IA , RI ’ Were un der the adminis-
trative control of ICAR. The research programme for each region
was prepared by a regional coordination committee headed by the
Agriculture Commissioner of India, and approved by the respective
Son 0 s d, o y f C nlflm ]tt h eeS H* A regi ° naI s£ation consisted of full-fledged
j,L j * f? breeding and genetics, agronomy, agricultural
chemistry and soil science, plant pathology and entomology.
Organisations for Crop improvement in India
Table 29.2. of Research
Location of centre Province
Research work on
Coimbatore Tamil N.do Cottoo, jowar, groontat
K£a S u, Karnataka gffiSS”
Sula kere n ha - ri f jowar
bulakere J Ragi gr0lmc ] nut
Rajendrasagar Andhra Pradesh Castor, groundnut
Mohoj atl 1 , , Cotton, jowar, groundnut
Monoi j Maharashtra Rabi, jowar
Junagarh "I Jowar, groundnut
Surat J Gujarat Cotton , jowar
Gwalior 1 Kharif jowar
Hoshangabad J Madhya Pradesh Linseed
Ajoaer Rajasthan Jowar, bajra
Kanpur Uttar Pradesh Indian mustard, bajra
Patiala Punjab Toria, taramira
Sirsa Haryana Cotton
IARI New Delhi Cotton, jowar, bajra, linseed
(fundamental research, on phy-
siology andjjcv togenefics ; linseed
breeding)
Initiation of All India Coordinated Research Projects
The concept of coordinated projects first developed in relation
to hybrid maize improvement. ICAR was interested in utilizing
r.Tfnl 0 ! m& T lm P r , ove , men£ as £ his approach was highly success-
ful m U.S.A. and several other countries. Rockefeller Foundation
Savely involved in crop improvement programmes in Mexico,
Central America and the Caribbean, was invited to assist in the
maize improvement programme in India. The Ministry of Food and
Rn^lf £ n re ’ 5 r0ve , rnment . of ^ dia > signed an agreement with the
5^ f « /°™dati°n in 1956. According to this agreement
Rockefeller Foundation was to assist in the development of (1) the
postgraduate school of Indian Agricultural Research Institute (IARI)
New Delhi, and (2) research programmes on the improvement of
tbf S r ( ,T a,Z & J ° V T r a nd bajra ’ initially). Two scientists from
CnlnmWn^fm . F ° uadatIon mmze Programmes in Mexico and
Columbia came to India to study the position of maize ctod and
submitted their report. This report was scrutinised by the Botany
Committee of ICAR and then by the Advisorv Board of the Council
and provided the basis for the coordinated maize project.
counts f aize . improvemen!: Programme, the entire
country was divided mtc major agrochmatic zones without anv
I^ rdt °p ate boundaries. Each zone was to have several research
•centres. Research centres present in » ctat* would he under the
Tamil Nadu
Karnataka
Andhra Pradesh
Maharashtra
Gujarat
Madhya Pradesh
Rajasthan
Uttar Pradesh
Punjab
Haryana
New Delhi
37 y
flani Breeding : Principles and , Methods
administrative control of that state. ICAR was to provide a Project
Coordinator who would visit all the centres under the project to
facilitate smooth running of the project. There would be annual
workshops of the principal research workers to review the progress
during the year and to formulate the research work for the next
year. "The ’Rockefeller Foundation agreed to provide the world
collection of germplasm, and to provide another Project Coordinator
In the, early stages of the programme ; this coordinator was to work
in close cooperation with the Project Coordinator appointed by the
Council. With this set up, the All India Coordinated Maize Improve-
ment Project was initiated in 1957. This was . a landmark in the
agricultural research in India ; a coordinated approach for the whole
country was initiated in place af the fragmentary and isolated research
programmes in the past .
The coordinated maize project proved to be the turning point
in research planning in agriculture. By 1961, new high yielding
maize hybrids became available as a result of the coordinated pro-
ject. Spurred by this success of the concept, ICAR decided in 1965
to initiate coordinated projects on other crops as well as in other
areas of research, e.g., animal husbandry, soil sciences etc. Within
3 years of this decision, 70 coordinated projects on various subjects
were launched, The coordinated projects accounted for 40 per cent
of the total outlay for agriculture in the Fourth Five Year Plan. The
progress of the coordinated projects was critically reviewed in the
Fifth Five Year Plan ; some projects were terminated, some were
merged with other projects, some projects were elevated to the level
of Project Directorates and some were changed to Coordinated
Programmes. In the Fifth Five Year Plan, there were 49 coordinated
projects and some coordinated programmes. The 22 coordinated
projects related to crop improvement are listed in Table 29.3.
Reorganisation of ICAR
In 1963, the Agricultural Review Team was appointed by the
Ministry of Food and Agriculture to scrutinise the organisation
of agricultural research in India. The team was headed by
Dr. Marion W. Parker of USDA (United States Department of Agri-
culture). The team submitted its report in March, 1964 . Based on the
recommendations of the team, ICAR was reorganised in 1966. ICAR
was made a fully autonomous organisation. Various research orga-
nisations under the Department of Food and Agriculture and under
the Central Commodity Committees were brought under the control
of ICAR. The Governing Body of ICAR was reorganised to make it
primarily a body of scientists and agriculturists. IARI, National
Dairy Research Institute and Indian Veterinary Research Institute
were made national institutes. A provision was made for the recruit-
ment of scientists through selection committees of ICAR. It was.
made a policy that an agricultural scientist would, be appointed as
the chief executive of ICAR with the designation of Director
General. In May 1965, Dr. B.P. Pal was appointed as the first
»
Organisations for Crop Improvement in India 571
Table 29,3. All India coordinated crop improvement projects
Name oj project Headquarters Remarks
Wheat
Rice
Pulses
Oilseeds
(a) Food Crops
Maize
Jowar
Barley
Millets
Forage crops
National seed
project
(if) Commercial Crops .
Sugarcane
Sugarbeet
Cotton
Jole and allied fibres
Soybean
Tobacco
Cotton project (assisted
by World Bank
(e) Horticultural Ctop s
Fruits
Citrus
Tuber .crops
'Potato'
Vegetables
Medicinal and aromatic
plants
Spices and cashewnut
Coconut and arecanut
Under- utilized and
under-exploited plants
Project Directorates
New Delhi
Hyderabad
Kanpur
Hyderabad
Coordinated Projects
New Delhi
Hyderabad
Kama?
Pune
Jhansi
New Delhi ..
Lucknow
Fantnagar
Coimbatore
Barrackporc
Indore
Anand (Gujarat)
Nagpur
Bangalore
Bangalore
Dholl (Bihar)
Simla
New Delhi
New Delhi
Kasaragod
Kasaragod
New Delhi
IARI Campus
Under CRR1, Cuttack
IARi Regional Research
Station, Kanpur
Project Coordinators :
Groundnut,' Rapeseed and
mustard, Sesamum and
niger, - Castor, md Sun-
flower
IARI Campus, first coor-
dinated project, started In
1957
IARI Regional Research
Station, Hyderabad
IARI Regional Research
Station, Kama!
MPKV, Rahuri, Pune
Campus
IFGRI, Jhansi
ICAR headquarters
HSR, Lucknow 1
G.B.RU.A. & T,
Fantnagar
Started in 1967
JARI, Barrackporc,
W.B.
National Research Centre
for soybean, Indore
Started in 1 970
Started in ■ 1976, Locat-
ed at OCR, Nagpur
II HR, Bangalore
II HR, Bangalore
RAU, Dholt Campus
Started in 1971, under
CPRI, Simla
IARI Campus
NBPGR, IARI campus
Under CPCRI,
Kasaragod
Under CPCRI,
Kasaragod '
NBPGR, IARI campus
572
Plant Breeding : Principles and Methods
Director Genera! of ICAR ; he was simultaneously Vice-President of
the Council. Four posts of Deputy Director General were created to
assist the Director General.
At present, the total number of institutes under the Council
is 33 ; those related to crop production and related subjects are listed
in Table 29.4.
Table 29.4. Central institutes engaged in research work on the various
aspects of crop production.
Institute
Location
Year established
Remarks
Indian Agricultural
Research Institute
(IAR 1)
New Delhi
1905 (shifted to
Delhi in 1936
from Bihar)
Teaching and Research ;
14 substations
Central Arid Zone
Research Institute
CCAZRI)
Jodhpur
(Rajasthan)
1959
Research on aridity in arid
and -semi-arid regions
Cotton Technolo-
gical Research
Laboratory
(CTRL)
Matunga
(Bombay,
Maharash-
tra)
1924 (transfer-
red to ICAR in
1966)
Evaluation of quality of
new strains ; 9 regional
units
Indian Grassland
and Fodder Re-
search Institute
(IGFRI)
Jbansi
(U.P.)
1962 (transferred
to ICAR in 1966)
Research on fodder and
forage crops
Indian Institute of "
Horticultural Re-
search (JIHR)
Bessara-
ghatta
(Bangalore,
Karnataka)
1967
Horticultural crops ; 3
stations
Jute Agriculture
Research Institute
(JAR!)
Barrack-
pore
(W.B.)
1953 (est. in
Dacca in 3939 s
under ICAR
since 1966)
Jute, mesta, ramie, sisal
and sunnhemp
Jute Technological
Research Labora-
tories (JTRL)
Calcutta
(W.B.)
1938 (under
ICAR since
1965)
Technological research on
jute and other fibres
Indian Lac Re-
search Institute
(ILR1)
Namkum
(near
Ranchi,
Bihar)
1925 (under
ICAR since
1966)
Research on Lac cultivat-
ion and processing
Central Plantation
Crops Research
Institute (CPCR1)
Kasaragod
(Kerala)
1970
5 regional stations, 7 re-
search centres, 2 seed
farms ; plantation crops
and tree spices
Central Potato
Research Institute
(CPRI)
Simla
(H.P.) y
1949 (est. at
Patna, shifted
to Simla in
1956)
Potato ; 7 regional stat-
ions, 4 seed multiplication
centres
Central Rice Re-
search Institute
(CRRI)
Cuttack
(Orissa)
1945 (under
ICAR since
1966)
Rice ; 4 substations
Organisations for Crop Improvement in India
573
Institute
Location
Year established
Remarks
Central Soil Sali-
nity Research Insti-
tute (CSSRI)
Kama!
(Haryana)
1969
Management of alkaline:
soils ; 1 substation
(a breeding section)
Indian Institute of
Sugarcane Re-
search (HSR)
Lucknow
(U.P.)
1952 (under
ICAR since
1969)
Sugarcane (breeding in-'
eluded)
Sugarcane Breeding
Institute (SBI)
Coimbatore
(Tamil
Nadu)
1912 (under
ICAR since
1969)
Sugarcane ; 2 substations
Central Tobacco
Research Institute
(CTRI)
Rajahmun- 1947 (under
dry (Andhra ICAR since
Pradesh) 1965)
Tobacco ; 6 stations
Central Tuber
Crops Research
Institute (CTCRI)
Trivandrum
(Kerala)
1963
Tuber crops other than
potato
Central Institute for
'Cotton Research
(CICR)
Nagpur
(Maharash-
tra)
1976
Cotton
Centra! Soil and
Water Conservation
and Training Insti- ’
tute (CSWCTI)
Dehradun
(U.P.)
1974
Research /training on soil
and water conservation
Central Institute of
Agricultural Engi-
neering (CIAE)
Bhopal
(M.P.)
1976
Agricultural and related
equipments
Vivekananda Parva-
tjya Anusandhan
Shala (VPAS)
Almora
(U.P.)
1924 (shifted to
Almora in 1936 ;
under ICAR
since 1976)
Agricultural research for
hills
National Bureau
of Plant Genetic
Resources
(NBPGR)
IARI cam-
pus (New
Delhi)
1976
Introduction and export
of germplasm
National Bureau of
Soil Survey and
Land Use
Planning
Nagpur
(Maharash-
tra)
1976
Detached from IARI
ICAR Research
Complex for
North-Eastern
Hills Region'
(RCNEHR)
Shillong
(Megha-
(laya)
1975
Agricultural and animal
sciences and fisheries ; 9-
siations
Central Agricul-
tural Research
Institute for Anda-
man and Nicobar
group of Islands
CARI (ANGI)]
F ori Blair
1978
Agricultural and animal
sciences and fisheries
574
Plant Breeding : Principles and Methods
Institute
Location
Years established
Remarks
National Academy
of Agricultural
Research Manage-
ment (NA ARM)
Hyderabad
(A.F.)
Training JARS recruits ;
previously Central Staff
College for Agriculture
Indian Agricui-t
mrai Statistics Re*
search Institute
CIASRI)
New Delhi
1959
Agricultural statistics
In June, 1972, the Government of India appointed a committee
to review the recruitment and the personnel policies of ICAR and
its institutes, and to suggest measures for their improvement. The
committee was headed by Mr. Gajendragadkar, retired Chief Justice
of India, and submitted its report in January, 1973. In view of
the recomendations by this committee, a Department of Agricul-
tural Research and Education was created in the Ministry of Food
and Agriculture in December, i973. The Director General, ICAR,
was made secretary to the new department. The Minister of Agri-
culture was designated as the President of the council, while the
Director General, ICAR, was made the Chairman of the Governing
Body. The Advisory Board and the Standing Committee were abo-
lished ; the functions of the Standing Committee were assigned to
Scientific Panels. The Scientific panels for different disciplines
consider and evaluate the suitability for financial assistance of
research schemes. An Agricultural Research Service was initiated for
the recruitment of scientific personnel under the Agricultural
Scientists’ Recruitment Board. A scheme for internal assessment
and promotion was initiated. The credit for these changes goes to
Dr. M.S. Swaminathan, the then Director General of ICAR.
The entire country was divided into 8 agroecoiogical zones.
For each zone, regional committees were set up ; the Director
General of ICAR was made ex-officio chairman of these committees.
The function of these regional committees is to review the status
of agricultural research and education in the concerned regions. The
Governing Body of ICAR is assisted by a Norms and Accreditation
Committee, which looks after the development of Agricultural
Universities and the grant of fellowships.
Development of Agricultural Universities
Before independence, higher education in agriculture was
virtually neglected. In 1948, there were 17 agricultural colleges in
the country, which were under the control of Director, Department
of Agriculture of the respective states. Colleges for animal hus-
bandry were separate from those for agriculture, and were governed
by the Director, Animal Husbandry of the states. Research and
extension were the responsibility of the agriculture and the animal
husbandry departments of the states. The organisation, staffing
pattern, pay scales of teachers and financial support (which was
Organisations for Crop Improvement in India
575
solely by the state through the respective departments) were not
suitable for a first grade education and training in agriculture.
The University Education Commission (1948-49) headed by
Dr. S. Radhakrishnan, recommended that rural universities should
be established. In 1950, Major H.S. Singh and Mr. A.N. Jha
(Chief Secretary and Development Commissioner, U P.) visited the
Land-Grant Universities of the United States. They advised the then
Chief Minister of U P., Pandit Govind Ballabh Pant, to set up such
a university, and he accepted their advice. This event may be
regarded as the one which led to the initiation of agricultural
universities. In 1955, the first Joint Indo-American Team was set up
which recommended the establishment of rural universities in each
of the states. The team felt that U P. (Tarai), West Bengal
(Haringhatta), Bihar (Patna), Orissa (Bhubaneshwar), Travancore-
Cochin and Bombay (Anand) states were suitable for starting such
universities.
Dr.H.W. Hannah prepared a bine-print for agricultural univer-
sities in 1956 ; this provided the basis for the proposal by the
Government of U.P. to the Central Government (in September,
1956) for starting an agricultural university near Rudrapur in the
tarai region of U.P. The Central Government agreed to the
proposal on an experimental basis. The second Joint Indo-American
Team was set up in 1959, which submitted its report in 1960. The
tea m recommended that the Agricultural Universities should be
autonomous ; should consist of colleges of agriculture, veterinary,
animal husbandry, home science, technology and basic sciences •
should have inter-disciplinary teaching programmes ; and should
integrate teaching research and extension. By 1961, there were
demands from many states for agricultural universities and the
Government of India accepted the organisation of a few more
agricultural universities during the Third Five Year Plan.
In 1960, the Government of India appointed a committee for
providing a model for the necessary legislation by the states for the
establishment of agricultural universities. The committee was headed
by Dr. R.W. Cummings and submitted its report in 1962. On the
basis of this report, ICAR prepared the model act for the develop-
ment of agricultural universities. During 1960-65, the fourth Five
Year Plan, seven agricultural universities were established in U.P.,
Orissa, Rajasthan, Punjab, Andhra Pradesh, Madhya Pradesh and
Karnataka. The United States Agency for International Development
(USAID) contributed significantly to the development of agricultural
universities through the Land-Grant Universities of U.S.A. This
assistance was in the form of training of Indian scientists in U.S.A,,
stationing of U.S. scientists for teaching and research in Indian
agriculture universities and a limited amount of equipments for
teaching and research.
The Education Commission (1964-66), headed by Dr. D.S. Kothari,
recommended that all aspects of agricultural research should’
576
Plant Breeding ; Principles and Methods
be the function of agriculture universities. Subsequently, the res-
ponsibility ♦for research was delegated from State Departments of
Agriculture to agricultural universities, but this change was not
uniformly implemented in every state. The Review Committee on
Agriculture Universities (1977-78), headed by Dr. M.S. Randhawa,
made many useful recommendations for the development of agri-
culture universities. It noted that the quality of leadership and the
financial ’ support from the state were crucial factors in the
development of agricultural universities. The committee suggested,
among other things, that the Director General, ICAR, and Chair-
man, University Grants Commission, should be members of the
selection committee for appointing the Vice-Chancellors for agri-
culture universities. The Model Act should be followed faithfully,
and states should accept the responsibility for developing agriculture
universities.
One of the original objectives of the ICAR was to undertake,
aid, promote and coordinate agricultural education in the country,
But this was not put into effective practice until the reorganisation
of ICAR in 1966. A full-fledged Division of Agricultural Educa-
tion was set up within the ICAR to fulfil this objective. The ICAR
has been crucial in the reorgonisation of agricultural education in
the country by providing the necessary guidance, financial aid
(Rs. 41 crores during 1974-75 to 1978-79), and schemes for improving
the quality of teaching and research, e.g., centres of excellence, higher
education in new areas. Professors of Eminence, faculty improve-
ment, scholarships and fellowships.
The total number of agriculture universities is now 27 (Table
29.5). The agriculture universities have contributed to a great
extent to agricultural education, research and development in the
country. Many improved varieties have been developed in the
agriculture universities and they have made numerous other
contributions to some of which we shall return later.
Table 29.5. Agriculture Universities in India.
University
Location Year esta-
blished
Remarks
Govind Ballabh Plant University
of Agriculture and Technology
Pantnagar
CU.P.)
1960
First agriculture
university ; single
campus
Rajasthan Agriculture University
(bifurcated from University of
Udaipur)
Bikaner
(Rajasthan)
1986
3 campuses
Orissa University of Agriculture
and Technology
Bhubane- 1962
sbwar (Orissa)
Single campus
Punjab Agriculural University
Ludhiana
1963
Single campus
Jawaharlal Nehru Krisht
Vishwa Vidyalaya
Jabalpur
(M.P.)
1964
9 campuses
Andhra Pradesh Agricultural
University
Hyderabad
1965
6 campuses
University of Agricultural
Sciences
Bangalore
(Karnataka)
1965
4 campuses
Organisations for Crop Improvement in India
577
University
Location
Yearesta- Remarks
wished
Mahatma Phule Krishi
Vidyapeeth
Punjabrao : , rishi Vidapeeth
Assam Agricultural University
Haryana Agricultural University
Marathvvada Agricultural
University
Rajendra Agricultural University
Rahuri }965
(Maharashtra)
Akola t 1959
(Maharashtra)
Jorhat (Assam) 1969
Hissar 1970
(Haryana)
Parbhani 1972
(Maharashtra)
Pusa 1971
iBihar)
4 campuses
7 campuses
Single campus
Bifurcated from
Single campus
5 campuses
Konkan Krishi Vidyapeeth
Da poll
(Maharashtra)
1972
4 campuses
Tamil Nadu Agricultural
University
Coimbatore
(T.N.) ,
1972
3 campuses
^Kerala Agricultural University
Vellanikkara
(Kerala)
1972
3 campuses
Gujarat Agricultural University
Dant'.wada
(Dist. Bankan-
tha, Gujarat)
1972
4 campuses
Vidhan Chandra Krishi Vishwa
Vidyalaya
Haringhatta
(Dist. Nadia,
1974
W,B.)
Chandra Shekhar Azad Univer-
sity of Agriculture and Techno-
logy
Kanpur
(UP.)
1975
2 campuses
Narendra Dev University of
Agriculture and Technology
Kumarganj
(near Faizabad,
1976
Single campus
U.P.)
Himachal Pradesh Krishi
Vishwa Vidyalaya
Palampur
(H.P.)
1978
2 campuses
Institute of Agricultural Sciences
Varanasi
1980
.. Within the' ;
(UP.)
Banaras Hindu
Birsa Agriculture University
Ranchi
(Bihar)
1980
University
Single campus
Sher-e- Kashmir University of
Agricultural Sciences and
Srinagar
(I&K)
1982
2 campuses
Technology
Dr v Y.S ; Parmar University
of Horticulure and Forestry
Palampur
(H.P.)
1985
1 campus
Indira Gandhi Krishi
Vishwa Vidyalaya
Raipur 1937
(M.P.)
I campus
Science s -
Dharwar
(Karnataka)
1987
378 Plant Breeding : Principles and Methods
THE ORGANISATIONS FOR CROP IMPROVEMENT
Organisation And Functions of ICAR
The Indian Council of Agricultural Research (ICAR) is
synonymous to agricultural research and education in the country.
The council is an autonomous society and has played a crucial role
In the development of agricultural reasearch and education. The
objectives of the council may be briefly summarised as follows :
to promote, guide and coordinate agricultural and veterinary
research and education throughout India, to train research workers
by offering scholarships, to serve as a clearing house of information
in regard to research and toadvise on agricultural and veterinary
matters generally, and to undertake the publication of scientific
papers, monographs etc.
The Minister and the State Minister of Agriculture and Irriga-
tion are the President and the Vice-President, respectively, of the
council. The Director General is the Principal Executive of the
council (Fig, 29.1) ; he is also Secretary to the Government of
India in the Department of Agricultural Research and Education.
He functions as the principal advisor to the government in the
matters related to agricultural and veterinary research and educa-
tion. The council functions through the following bodies.
won!
mom
Mom
in p
Mom
and v
Lacat
mens*
carry!
they a
(idem!
F*g«|
INDIAN COUNCIL 6ff AGRICULTURAL -RESEARCH
PRESIDENT
(MINISTER FOR AGRICULTURE a IRRIGATION.)
fSTATE MINISTER FOR AGRICULTURE 6 IRRIGATION)
DIRECTOR GENERAL
(CHIEF EXECUTIVE)
REGIONAL' committees]
standing FINANCE committee
INSTITUTES OR
NATIONAL
IMPORTANCE
t . INDIAN AGRICULTURAL
RESEARCH INSTITUTE,
NEW DELHI
2. INDIAN VETERIWAWV
RESEARCH INSTfrwS,
12ATNAGAR
3 . NATION*!. DAIRY
' research
KARNAL
COORDINATION
COORDINATED CROP
IMPROVEMENT PROJECT®
CROP INSTITUTES
DEVELOPMENT OF
AGRICULTURE
' UNIVERSITIES
NATIONAL ACADEMY of
AGRICULTURAL RESEARCH
& MANAGEMENT
Organisation of ICAR. Details are shown for the organs related
to cron in3Dmvpmon+
DEPUTY
DIRECTOR -GENERAL ( DOG)
(CROP SCIENCES)
- : f
DDG (SOIL
SCIENCES)
ODG (ANIMAL
SCIENCES)
DDG f AGRI-
CULTURAL
EDUCATION)
i Organisations for Crop Improvement in India 579
Governing Body. The Director General presides over the Go\ era**
ing Body, which is the chief executive and decision making autho-
rity. It consists of agricultural scientists and persons with a know-
ledge of and interest in agriculture. It makes decisions regarding
policies, research projects and schemes, and controls the budget.
Standing Finance Committee. Standing Finance Committee is a sub-
committee of the . Governing Body and is presided over by the
Director General. It examines budget proposals before they are
put before the Governing Body.
Norms And Accreditation Committee. The committee consists of 5
Vice-Chancellors of agriculture universities nominated by the Presi-
dent of the Council, and is headed by the Director General. It is
responsible for the development of agricultural universities, and
maintenance of standards of education in agriculture and animal
sciences.
Regional Committees. The conntry is divided into 8 agroclimatic
zones ; each zone has a regional committee headed by the Director
General. These committees review the status of agricultural research
and education in the respective zones and make appropriate
recommendations.
Scientific Panels. There are 18 scientific panels for individual disci-
plines, and 5 interdisciplinary panels. The scientific panels scrutinise
research schemes and projects of the disciplines concerned and advise
the Governing Body on the technical matters related to research and
■education.
The council has four Deputy Director Generals for (I) Crop
Sciences ; (2) S.oils, Agronomy and Agricultural Engineering,
(3) Agricultural Education ; and (4) Animal Sciences. The DDGs are
assisted by Assistant Director Generals and other Technical Officers.
The DDG (Crop Sciences) is responsible for all the projects related
to crop improvement.
The council receives a lump sum grant from the Government
of India, and the receipts of the agricultural produce cess fund. The
council has a separate service cadre, Agricultural Research Service
(ARS) for its scientists. The ARS cadre is designed to encourage
research activities. Every 5 years, an assessment of the performance
of each scientist is made and the scientists are granted either
promotion or increments for their achievements.
The crop improvement activities in the country are primaril}
confined to government and semigovernment institutions ; there are
some private organisations engaged in crop improvement activities.
All the crop improvement activities (except of private organisations;
in the country are directly or indirectly supported, supervised and
coordinated by ICAR, There are four main channels by which th(
council is involved in crop improvement activities : (1) centrr
institutes on crops, (2) agriculture universities, (3) coordinated cro
imorovemeht projects, and (4) ad hoc research schemes.
■ . won]
; moth
■ Mom .4
in p
Mom
and i?
■ ■■Lpeat -y
monsi
carryi
they a
(ident JL
P* 8&j 1
chr^BB
j-
Ml!
*j4'-
580 Plant Breeding : Principles and Methods
In addition, the council awards a number of prizes in the
recognition of outstanding research achievements. The awards are ;
Rafi Ahmad Kidwai Memorial Prize (1956, 11 awards of Rs. 10,000
each, every 2 years). Dr. P.B. Sarkar Endowment Prize (1971. ’one
award of Rs. 5,000, once in every 3 years), ICAR award for Team
Research (1974, 3 awards, every 3 years), Hari Om Ashram Trust
Award (1975, 2 awards of Rs. 10,000 each, every year). Dr. R.D
Asana Endowment Prize (1974, one award of Rs. 2,000, every 3
years) and Jawaharlal Nehru Award (1972, 5 awards of R s . 5 000
each, every year).
The council also provides financial assistance to registered
societies in agriculture and animal sciences for the publication of
research journals, for conferences, seminars and symposia, for
summer institutes and short courses, travelling expensesfor attending
international conferences etc. . •
Central Institutes for Crop Improvement
A number of research institutes are engaged in crop improve-
ment (Table 29.4). These institutes are under the control of ICAR
Some of the institutes work or. a single crop, e.g.. Central Rice
Research Institute, while some work on more than one crop, e g
Central Plantation Crops Research Institute. A brief account of
these institutes is given below.
Indian Agricultural Research Institute (IARI). An Agricultural Re-
search Institute was established in Pusa in 1905. Pusa is a village-
in Bihar, then in the province of Bengal. In 1934, the institute
building suffered damage due to earthquake ; the institute was
transferred to New Delhi in 1936. In 1947, the name of the insti-
tute was changed to the present Indian Agricultural Research Insti-
tute. The institute is primarily an educational institution. A two year
postgraduate diploma was started in 1923, and in the year 1958 it
was granted the status of a deemed university by the University
Grants Commission.
The institute has an unparalleled history in the field o'" agri-
cultural research. Many central institutes owe their origin to
research projects, initiated at this institute, e.g., SBI, CTRI, CPRI, :
Indian Lac Research Institute etc. A more recent example is the
NBPGR which began as a scheme for plant introduction in the
Botany Division of the institute. The institute has 14 stations
scattered throughout the country. IARI has developed many high
yielding varieties of wheat, jowar, maize, bajra, linseed, arhar, vege-
tables etc. In addition, basic aspects of crop improvement . have been
extensively investigated at the institute.
Sugarcane Breeding Institute (SBI). The Sugarcane Breeding Institute
was established in 1919, and in 1924 was taken over by the Govern-
ment of India from the Madras Government. The institute became
a part of ICAR in 1962. It has 4 stations at Karnal (Haryana),
Cannanore (Kerala), Lucknow (U.P.) and Motihari (Bihar). The
.Organisations for Crop Improvement in India 581
institute is responsible for breeding improved varieties of sugarcane
.and to conduct basic research on genetics and cytogenetics of sugar-*
•cane; it is also responsible for research on other aspects of
'sugarcane cultivation. In Coimbatore, sugarcane Sowers freely and
sets plenty of seed.
The institute has evolved nearly 2,000 varieties of sugarcane ;
all Co varieties owe their origin to this institute. These varieties
have been extensively used in the country and in some 26 other
countries. The sugarcane varieties are produced from complex crosses
between S. officinamm , S. spontaneum and 5. barbed . Crosses
between S. officinamm and S . spontaneum were initiated by
Dr. C.A. Barber (1912-28), the first Head of the Institute ; after him
the Indian canes have been named S. barbed. The institute has a
large germplasm collection with 2,800 entries. A notable achievement
of the institute in distant hybridization is a cross between sugarcane
and maize, and another between sugarcane and jowar. In 1974, a
National Hybridization garden was established in which participating
•states make desired crosses. Several somaclonal variants from tissue
culture have been released as varieties ; this achievement is unique
in this country.
Indian Institute of Sugarcane Research (IISR). The institute was
established in 1952 and was taken over by ICAR in 1969. In 1970,
a Botany and Breeding section was added for sugarcane improve-
ment The institute has made notable contributions in the methods
for sugarcane cultivation. The coordination unit of the All India
Coordinated Sugarcane Improvement Project is located at this insti-
tute. It also coordinates the All India Coordinated Seed Production
Programme for Sugarcane. The institute has 5 stations at Golago-
karaooath, Captaioganj (U.P.), Dimapur (Nagaland), Pravarnagar
(Maharashtra; and Rudrur (A.P.).
Cotton Technological Research Laboratory (CTRL). It was established
in 1924, and was transferred to ICAR in 1966. The laboratory eva-
luates the quality of new cotton strains ; in addition, it conducts
■basic research on quality of cotton. It is recognised by the University
of Bombay for postgraduate , and/or Ph.D. training in Textile
Physics, Biophysics, Textile Technology and Physical Chemistry,
The laboratory has 11 regional units at important cotton
breeding centres at Coimbatore, Dharwar, Guntur, Hissar, Indore,
Ludhiana, Handed, 'Sriganganagar, Surat etc. Prelimmary screening
of new strains is done at the regional units (over 10,000 samples
every year). The laboratory screens over 3,000 samples every year
for fibre and microspinning properties and 500 samples are subjected
to full spinning tests. The laboratory is the centre foi coordinating
the technological work under the All India Coordinated Cotton
Improvement Project. '
Central Institute for Cotton Research (CICR). The institute was
established in 1976. The Coimbatore centre of PIRRCOM, established
in I960, was attached to this institute. The institute is designed as a
centre for germplasm collection of cotton from which advanced
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582 , P/a/ 2 ? Breeding •} Principles end Methods
breeding material would flow to cotton breeders for location-
specific selection. The institute has collected so far 4,000 entries of
cultivated as well as 28 wild species of Gossypium. The Coimbatore
centre has evolved many high yielding varieties of cotton. Some
outstanding examples arc Sujata and Suvin varieties which are
comparable to the Egyptian cotton varieties like Giza 45 in their
fibre quality. PRS 72 is a short branched, early maturing variety
suited for intensive cropping and MCU 5- VT (earlier MCU 5-WT>
is resistant to Verticillium wilt. Many other varieties have been
developed by the Coimbatore centre. The institute also conducts
research on other aspects of cotton cultivation.
Jute Agricultural Research Institute (JARI). A Jute Agricultural
Research Laboratory was established in Dacca in 1939, which was
shifted to Barrackpore during the partition; JARI was formally
established at Barrackpore in 1953. In I960, the work of JARI was
extended to cover mesta, ramie, sisal and sunnhemp, and a pro-
gramme of seed multiplication was initiated. The Institute has a
Ramie Research Station in Assam and a seed multiplication farm at
Panagarh, Bud Bud.
The institute is responsible for the improvement of jute ramie
mesta, sisal and sunnhemp and for other relevant researches on
these crops. The institute has evolved a large number of improved
varieties of both Corchorus olitorius (JRO) and C. capsularis ORO
JRC212 and JRC 321, evolved during late 1940s are still popular
varieties ; JRC 321 is a selection from a local variety Hewti
JRC 321 is early and suitable for lowland conditions. Resistance to
premature flowering and to pod shattering has been transferred from
Sudan Green, and varieties like Basudev and JRO 524 have been
developed. JRO 524 is reported to have the best fibre quality and
easy retting. The first ever monoploid in jute was also reported from
this institute. ■ .
Jute Technological Research Laboratories (JTRL). The laboratory
was established in 1938 and was transferred to ICAR in 1965.
Among the responsibilities of the laboratory are included : technolo-
gical research on jute and other long vegetable fibres, and evaluation
of fibre quality of new strains. The technological unit of the All
KteisteaSa,JTRL C * f “ r ° r “J
Central Rice Research Institute (CRRI). CRRI was started in 1946
and was transferred to ICAR in 1966, Dr. K. Ramiah, a well known
sfrbt? 1 ft S tfe r*. was i ts Sr f director - The institute has stations at
Simhgoda (rice and ragi), Kalimpong (W.B.), Nellore (A. P. ; both
foT)disease and pest forecasting) and at Hazaribagh (Bihar, rainfed
coordinates collection and evaluation of rice aerm-
plasm throughout the country. It also coordinates the rice improve-
ment work m India through the All India Coordinated*^ Rice
Organisations for Crop Improvement in India
583
Improvement Project (AICRIP). It is recognised by FAO as a world
centre for rice germpiasm collection ; the Institute has 15,000
entries. Medium-term storage facilities for germpiasm collections
are being developed. In addition, the institute carries out research
on genetics, cytogenetics and all the important aspects of rice culti-
vation and improvement. The contribution of CRRI is summed up
in the observation of Dr. Randhawa that ‘rice research in India
closely follows the establishment and growth of the Centra! Rice
Research Institute at Cuttack’. The present-day popular varieties
like Jaya, Padma, Bala, Vijaya, Sona, Yard, Shakti etc. • are some of
the notable achievements of CRRI and AICRIP.
Central Tobacco Research Institute (CTRf). The institute was initia-
ted in 1947 ; it was brought under ICAR in 1965. The CTRX has 6
stations: Vadasandur (T.N.) for cigar, cheroot and chewing tobacco;
Pusa (Bihar) for hookah and chewing tobacco ; Dinhata (W.B.) for
wrapper and hookah tobacco ; Guntur (A.P.) for cigarette tobacco ;
Hunsur (Karnataka) for diseases of hue-cured tobacco ; and
Kandukur (A.P.). The All India Coordinated Tobacco Improvement
Project is located at CTRL The institute is responsible for tobacco
improvement and other related researches on tobacco cultivation. A
number of high yielding varieties have been developed by the
institute, e.g. 9 Kanakprabha and Dhandavi flue-cured tobacco, and
DP 401 chewing tobacco etc.
Central Potato Research Institute (CPRI). CPRI was first established
at Patna in 1949 and was transferred to Simla in 1956. The institute
has regional stations at P atna, Babugarh (U.P.). Rajgurunagar (near
Pune, Maharashtra), lullund'hur (Punjab), Shillong (Meghalaya) and
Ootacamund (T.N.) which serve as testing centres for the breeding
materials developed oi CPRI. It has wart testing centre at Darjee-
ling, and four stations for seed' production at Fagu (H.P*.), lullundur
(Punjab), Daurala and Kodaikanal. The All India Coordinated
Potato Improvement Project is located at CPRI.
The institute is devoted to basic and applied research on
potato. One of its objectives is to evolve improved varieties of
.potato for the entire potato growing area of the country, and produc-
tion and multiplication of disease-free seed. The institute has deve-
loped 16 high yielding disease resistant varieties, and has developed
the "seed-plot technique’ of seed multiplication for the production of
disease-free seed in the plains. It has a large collection of potato
germpiasm. The CPRI has collaborative programmes with the
international Potato Centre, Lima (peru) and with the Scottish
Plant Breeding Station, Edinburgh.
Centra! Taber Crops Research Institute (CTCRI), . The institute was
established in 1963 at Sreekaryam near Trivandrum. The objective
of the institute is to conduct basic and applied research on tuber
crops other than potato, viz., cassava (tapioca), sweet potato*
Dioscorea , Amorphophalius , Colocasia 5 Coleus etc. The Institute has
h cassava research project financed by the International Development ■
584
Plant Breeding : Principles and Methods
Research Centre, Canada. The institute is collecting indigenous and
exotic germplasm of the root and tuber crops listed above. CTCRI
has developed high yielding disease resistant varieties of cassava,
e.g. 9 H 97, H 165, H 226, Sree Sahaya and Sfari Vishakham ; of
sweet potato* e.g. 9 H 4 1 , H 42, H 268, H- 633, H 620, OP 1 and OP 2
(OP varieties developed from open-pollinated seed) ;• and Colocasia ,
e.g.y Kassibugga and Narkatia.
Indian Institute of Horticultural Research (IIHR). The Institute was
established in 1967 at the site of the National Hortorium at
Hass? raghatta, near Bangalore. It has three stations : Citrus Experi-
ment Station in Gonicoppal, Horticultural Experiment Station in
Chethalli (both in Karnataka) and Central Mango Research Station
at Rehmankhera near Luckow (U.P.). The responsibility of the
institute is basic and applied research on fruit crops* e.g^ Citrus ,
mango, grapes etc., and on vegetables. Four promising grape hybrids
have been developed which are to be released shortly, e.g. t C 36-16,
B 11- 3, C 2-5 and B 42-23.
Central Soil Salinity Research Institute (CSSRI). The institute was
established in 1969 to conduct research on reclamation and manage-
ment of salt affected (sodic) soils for crop production. The institute
has a station at Canning (W.B.). It has three divisions : ‘ Soils and
Agronomy, Genetics and Plant Physiology, and Engineering. A large
. number of crop varieties have been screened at the institute and a
number of relatively salt tolerant varieties of different .crops have
been identified. Some of these varieties are listed below.
Rice (early maturing) Pusa 2-21
Rice (late maturing) Java, PR 8
Wheat HD 1982 (Janak), WH 157,
WL 71 ?, HD 155 3, HD 2009
Barley DL 70, DL 36, B 5105, K 153,
K 198, BHS 24
Bajra PHB 13, HB 3, HB 4
Central Arid Zone Research Institute (CAZRI). The institute was
started in 1959 when the Central Research Farm at Jodhpur was
reorganised as CAZRI. The Institute is devoted to the develop-
ment of relevant technologies for a belter management of desert and
the basic research necessary for the above. The institute is not only
multidisciplinary, it also conducts research on animals. The crop
improvement activities of the institute are mainly introduction of
forest species from foreign countries, evaluation of varieties of
various fruit crops and a main centre of the All India Coordinated
Millet Improvement Project. Among the introductions, some species
of Eucalyptus and Acacia are suitable for desert afforestation ;
Eucalyptus virdis is promising for oil production and Simmondsia
chinensis (introduced from Arizona, U.S.A.) for wax extraction.
Organisations for Crop Improvement in India
( Indian Grassland and Fodder Research Institute (IGFRI), The insti-
tute was established in 1962 and was transferred to ICAR in 1966.
y IGFRI aims at developing technology for maximisation of fodder
| and forage production from various types of soils. Including alkaline
and saline soils, and under various levels of management, and to
■transfer this technology to farmers it has a division of plant improve-
ment. The institute also offers training in the production and
utilization of fodder.
National Bureau of Plant Genetic Resources (NBPGR), The bureau
was established* in 1976 when the Plant Introduction Division of
IARI was reorganised. The organisation and the function of the
bureau have been described in some detail in Chapter 2. Further
description will not be given here, except to note that the burea is
I the sole agency for the introduction and export of germplasm of all
plant species.
W:
The Central Plantation Crops Research Institute (CPCRl). The
institute was established ' in 1970 by uniting the Central Coconut
Research Stations at Kasaragod and Kanyagulam, and the Central
Arena nut ^Research Station at Vittal. The institute has 5 regional
stations* 7 research centres and 2 seed production farms. It is respon-
sible for basic and applied research on coconut, arecanut, cashewnut,
cacao, oil-palm, pepper, cardamom, ginger, turmeric and tree spices.
It is the headquarter for the All India Coordinated Improvement
Project on coconut, arecanut, spices and cashewnut. The institute
has developed the T X D and D X T hybrids of coconut that are
high yielding and early bearing. In addition, the institute has a long
list of achievements to its credit.
Vivekananda Parva%a Krishi Anusandliasi Shala (VFKAS), The"
Vivekananda Laboratory was started in 1924 in the kitchen room of
a- house in Calcutta by Prof. Bosi Sen. The laboratory was shifted to
Almora (U.P.) in 1936, In 1959, the laboratory was taken over by
the Department of Agriculture, U.P,, and was finally transferred to
ICAR in 1974 under its present name. The initial work was limited
to the physiology of living plant cells. But at present the anusandhan
shala has sections of physiology, genetics, pathology and chemistry.
ICAR Research Complex for North-Eastern Hills Region (RCNEHR).
The research complex was established in 1975 in order to fulfil the
research and development needs of the North-Eastern parts of the
country, viz., Meghalaya, Tripura, Manipur, Arunacha! Pradesh,
Nagaland, Mizoram, Sikkim, and the North Cachar and Mikir Hills
• of Assam. The headquarters of the complex is in Shillong (Megha-
laya), and it has 9 stations spread over the areas mentioned above*
* The responsibility of the complex covers the entire range from
agriculture, animal sciences to fisheries.
Central Agricultural Research Institute for Andaman And Nicobar
Group of Islands (CARS). The institute was initiated in 1 973 with its
.headquarters »t Port Blair. The responsibility of th* institute cover
586
Plant Breeding : Principles and Methods
the whole range of agriculture, animal sciences and fisheries. A
number of varieties of several crops have been evaluated for
their adaptability to local conditions. Feasibility of potato cultivat-
ion is also being investigated in some areas.
Agriculture Universities
Agriculture universities are semi-autonomous institutions
supported by the respective state governments and the ICAR, but
are deemed to be the responsibility of the state governments after
one plan period ; they receive financial assistance from ICAR for
specific projects. The Agricultural universities have inherited from
the Departments of Agriculture of the respective states the responsi-
bility for agricultural education, research and extention ; the
research stations and the staff thereof have also been transferred to
them. In some states, this change has neither been smooth nor
complete. At present, the number of agriculture universities in
India is 3 (Table 29.5). It is planned that each state should have
one agriculture university ; some states have more than one
university, e.g., U.P. and Maharashtra have 4 agriculture univer-
sities each.
Agriculture universities are responsible for research and exten-
sion activities of the entire state (or part of the state in U.P. and
Maharashtra). The research activities in the universities are looked
after by (1) teaching staff (primary duty teaching), (2) research staff
(primary duty research) at the main campus" as well as at the
regional stations, (3) staff of ICAR coordinated projects and
(5) research personnel in ad hoc schemes. In addition, students carry
out the major portion of basic investigations.. Agriculture univer-
sities were established on the concept of integrated teaching,
research and development. This integration is sought to be achieved
in two ways : each staff member is transferred to teaching, research
or extension positions, and each scientist is expected to devote some
time to teaching, research and extension with a major emphasis on
the primary duty.
Agriculture universities have contributed a great deal to crop
improvement activities in the country. A large number of high yield-
ing varieties of a number of crops have been developed by the
agriculture universities, e.g. 9 in maize, jowar, bajra, wheat, rice,
pulses, fruits and a number of other agricultural, and horticultural
crops. The examples are too numerous to be listed ; the . student
has only to look into the extension and research literature put '
out by his own university to come across a number of notable
contributions.
All India Coordinated Crop Improvement Projects ,
The All India Coordinated Crop Improvement Projects (Table
29.3) provide an efficient channel for m'ultilocatipipt '.testing' ; of:: newly
evolved strains by the agricultural universities and the* central instil-
tutes. In addition, the bre^ders/geneticists employed in the projects
Organisations for Crop Improvement in India
Functions of Coordinated Projects for Crop Improvement. The
coordinated projects serve two basic functions.
1. To evaluate the materials generated by central institutes and
agriculture universities under a wide range of agroclimatic
■ conditions and under uniform management
■ ,
may develop their own materials for inclusion in these tests. The
coordinated approach was first used in this country before anywhere
else in the world. Each coordinated project is headed by a project
coordinator and has a number of centies. The centres are located in
agriculture universities and central institutes ; sometimes they may
be located in traditional universities, institutions of other scientific
organisations, and in private institutions. The locations for centres
of coordinated projects are selected on two considerations : the im-
portance of the crop in the region, and the availability of infra-
structure for the project. In some cases, the council may develop the
infrastructure as alternative locations may not be available. The.
coordinated projects are financed by ICAR on 100% basis, but in
the case of agricultural universities 25% of the expenditure is borne
by the state. At present, the coordinated projects are grouped into
three categories : project directorates, coordinated projects and
coordinated programmes. The nature o Reactivities of all the three
categories of projects are the same, only the scope and the magnitude
differs.
Project Directorate. A project directorate is headed by a full-time
Project Director who is assisted by a number of Associate Project
Directors or Associate Project Coordinators and a group of scien-
tists. He also has a coordinating unit with supporting staff. The
project directorates perform the functions of the coordinated projects.
In addition, they maintain germplasm, organise off-season nurseries,
monitor pests and diseases and make forecasts etc.
Coordinated Projects. A coordinated project is headed by a full-
time Project Coordinator who is assisted by several Zonal Coordi-
nators and a Principal Investigator for each discipline. The , coordi-
nating unit consists of supporting personnel. The functions of the-
Project Coordinator are to plan, guide, supervise, coordinate and
monitor the programmes of the research work under the project.
Coordinated Programmes. Coordinated programmes are smaller than
coordinated projects ; they are headed by a Principal Investigator
(not a Project Coordinator), and do not have a coordinating unit
They perform the same functions as of the coordinated projects.
The bead of a coordinated project (Director/Coordinator/
Principal Investigator) is selected on the basis of his scientific achieve-
ments. He should be able to command respect from" his colleagues
and be able to guide and supervise the research activities of the pro-
ject., The’ coordinating units of the projects are located in agriculture
universities or in central crop institutes. .
.588
Plant Breeding : Principles and Methods
2. To make recommendations on the suitability of new strains for
release as varieties. This is done after a thorough testing for
all the . important characteristics of the new strains, e.g. 9
yielding ability, disease resistance, quality etc.
In addition, the coordinated projects provide a forum to scien-
lists working on related problems to exchange their views, a means
of critical review and replanning the research programmes and an
opportunity for the scientists under the project to initiate breeding
programmes of their own.
Factors Responsible for The Success of Coordinated Projects. The
coordinated projects have been extremely successful. A large number
of high yielding, disease resistant varieties have been released through
them. The projects are now placing a greater emphasis on minor
millets, pulses and oilseeds, and on the improvement of protein
content and quality. Rut the chief objective of the projects remains,
and perhaps shall remain in the future, the development of high
yielding and disease resistant varieties of various crops. The factors
responsible for the success of coordinated projects are briefly
summarised below.
1. The country was divided into eight zones on the basis of soil
and climate without any reference to political boundaries.
’ This led to a cooperation between scientists working in agri-
culture universities and central institutes. This has been able
to generate considerable enthusiasm among the workers, thus
eliminating isolation.
2. Appointment of full-time coordinators of high competence
has been responsible for a uniform experimentation of high
order.
3. A world collection of germplasm was made available to the
breeders at the beginning of the projects. This provided the
wide genetic base required for effective crop improvement
programmes.
4. The annual (in some cases half-yearly, e.g., rice, pulses) work-
shops provide an excellent forum for discussion, critical review
and repatterning of programmes of the projects. This has
enabled the projects to remain flexible and to make necessary
changes as needed.
5. A close interaction between scientists working in different
disciplines is an integral pan of the coordinated projects.
This has enabled the breeders to perform then ta c V<? more
easily, and the evaluation of new genotypes is more speedy
and accurate.
.Ad hoc Research Projects 1 :
ICAR supports on 100 per cent basis ad hoc research projects
on various asnects of agricultural ««imal sciences. These
Organisations for Crop Improvement in India
schemes are short-term projects, the period not exceeding 5 years in
any case. # The council provides full recurring expenditure, but not
nonrecurring expenditure, except in a few exceptional cases. The
council receives funds for ad hoc projects from the agricultural
produce cess' fund (a certain percentage) and from various internat-
ional agencies, e,g., UNDP, IDRC, SIDA, DANIDA etc.
Proposals for ad hoc schemes may be submitted by scientists
throughout the year. The scientific panels of the respective discip-
lines meet twice a year (May- June and N o vem ber-December) and
critically review the proposals. Research schemes approved by the
scientific panels are put before the Standing Finance Committee for'
its consideration. They are finally approved by the Governing Body
of the ICAR. Ad hoc schemes are sanctioned to agricultural univer-
sities, central institutes, traditional universities and private and
public institutions. -
The ad hoc schemes, by their nature, are not designed for crop
improvement per se. Instead, they are meant for solving specific
problems of crop improvement, which is expected to be achieved in
a shprt period of time. Such schemes generally relate to basic aspects
of crop improvement, that is, genetic and cytogenetic studies.
SUMMARY
Agricultural research had its beginning in 1871 when a Department of
Revenue, Agriculture and Commerce was created by the Government of India.
The first scientific staff in the department was appointed in 1982. Between 19 d
and 1905, six agricultural colleges were started ; in 1905, an Agricultural
Research Institute (now I AM) was established at Pusa, Bihar. ICAI was
established in 1929. Before the establishment of ICAR, and upto 1965, a
number of Central ' Commodity Committees looked after research and deve-
lopment of specific crops. These committees had established some of the
present-day central institutes. In 1956* ICAR, in cooperation with cotton and
oilseeds committees, started PIRRCOM centres which later gave way to the
coordinated projects. The first All India Coordinated Research Project was
initiated io 1957 for exploitation of hybrid vigour in maize. The success of
this project led to the establishment of several other coordinated projects ; at
present 26 coordinated crop improvement projects are in operation. The
ICAR was reorganised twice Jin 195 $ and in 1973) leading to its present set
•up.. The first agriculture university was established at Pantnagar (U.P.) in
I960, which stimulated establishment of agriculture universities in other states.
At present, there are 23 agriculture universities.
ICAR is the premier body for agricultural research and education in
India. The council functions in crop improvement through (1) centra! crop
research institutes (total number 20), (2) agriculture universities (total number
23) , (3) coordinated crop improvement projects (total numbers 26) and (4) ad
hoc research schemes. The central institutes and agriculture universities gene-
rate new strains which are evaluated in the coordinated projects. The ad hoc
schemes are concerned with basic aspects of crop Improvement and with
solving of specific problems. The coordinated projects are a landmark in
agricultural research in the country..'
QUESTIONS
1. Compare the following : (0 coordinated projects and project directo-
rates. (//) coordinated and ad hoc projects. <///) CPRI and CTCRf,
(iv) IARI and ICAR, and (v) agriculture universities and central insti-
tutes.
Plant Breeding : Principles
Briefly outline the initiation of coordinated protects List -
and discuss the factors responsible for their success. '
Sint in b lSdia CC ° Unt ° f ^ hist0ry of a encalt wa l research
Briefly outline the establishment and development of 1C
objectives and d.scuss its role in agricultural fesearch anc
Give a list of central institutes for crop improvement Br
the organisation and functions of any two oftfae central ins
Write short notes on the following * •,
rotef/vwT uaiversiti «’ i«0 PiRRCOM rontres““ ) pro
[m^ovimen t : CPrOJeCtS ’ ™ ICA *> and cen’trkl inftk
Briefly describe the history of development, functions and c
of agriculture universities to agricultural researching Jiu„L
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CHAPTER 30
International Institutes, for
Crop Improvement
In .addition to the national crop improvement programmes,
there are ten international institutes and one international agency
concerned with improving the agricultural production. Eight of the
ten institutes and the international agency are directly or indirectly
involved with crop improvement work. These institutes and the
agency supplement the national efforts. The international institutes
are scattered around the world and are situated in tropical countries.
The tropical countries constitute the developing countries or the
‘third world’. The agriculture in these countries is not well developed
and crop improvement work is generally not highly advanced.
They are characterised by poor crop yields due to poor crop varieties
and, more particularly, poor crop management. The main objective
of these institutes is to increase agricultural production of tropical
countries through applied research coupled with extension and edu-
cational activities. The functioning of these institutes is supported
and supervised by the Consultative Group for International
Agricultural Research ( CGIAR ).
The CGIAR was established in 1971 by the joint efforts of
Food and Agriculture Organisation {FAQ), the World Bank and the
United Nations Development Programme (UNDP). The CGIAR is
financially supported by sponsors, governments, development banks,
foundations and some other sources. It initiated and established six
of the eleven international institutes, and at present it financially
supports and supervises all of them. Periodically, CGIAR assesses
the progress and the programmes of these institutes with the help
of internationally recognised experts in the field of agriculture. The
various international institutrs are listed below.
1. International Rice Research Institute (IRRI), Los Banos.
Philippines.
2. Centro International de Mejoramiento de Maiz y Trigo
(CIMMYT), (International Centre for Maize and Wheat
Improvement), el Baton, Mexico.
3. Centro International de Agricultura Tropical (CIAT), (Inter-
national Centre for Tropical Agriculture). Palmira, Colombia.
4. International Institute of Tropical Agriculture (IITA), Ibadan,
Nigeria.
592 ' Plant Breeding '.Principles and Methods
5. West African Rice Development Association (WARDA),
Monrovia, Liberia.
6. Centro International de Papa (CIP), (International Centre for
Potato), Lima, Peru.
7. International Crops Research Institute for the Semi-Arid
Tropics (ICRISAT), P.O. Patancheru, Hyderabad, India.
8. International Laboratory for Research on Animal Diseases
(1LRAD), Nairobi, Kenya.
9. International Board for Plant Genetic Resources (IBPGR) p
Rome, Italy.
ID. International Livestock Centre for Africa (ILCA), Addis
Ababa; Ethiopia.
1L International Centre for Agricultural Research in Dry Areas.
(ICARDA), Aleppo, Syria.
International lice Research Institute (IRRI)
IRRI was initiated in I960 jointly by Ford and Rockefeller
Foundations, This institute is concerned with the improvement of
tropical rices. The emphasis of the institute is to develop genotypes
suited to tropical Asian countries. Research on other aspects of rice
jmoduction is .also carried out by the institute. In addition, it main-
tains a huge collection of rice germplasm consisting of more than
42,000 entries. IRRI has a sophisticated cold storage facility for
long-term storage of rice germplasm. It has a computerised system
of germplasm storage that greatly facilitates the retrieval of desired
germplasm. The semidwarf rice varieties ’were first developed by
IRRI from where they were introduced in India.
Centro International de Mejoramiento de Maiz y Togo (CIMMYT)
CIMMYT was established in 1966 by the joint efforts of Ford
and Rockefeller Foundations on the basis of an earlier pragfamme.
The earlier programme was initiated by Rockefeller Foundation in
collaboration with the Ministry of Agriculture of Mexico in 1943.
The primary responsibility of CIMMYT is the improvement of
maize and wheat. But it is also engaged in the improvement work,
on other cereals, such as Triticale , barley and jowar. In India,
contributions of CIMMYT are most apparent in the case of wheat ;
Mexican semidwarf wheats are notable examples. CIMMYT has
also been involved in the Triticale programme of the country. This
institute has opted to work on open-pollinated maize varieties
instead of hybrid or synthetic maize varieties for obvious reasons.
CIMMYT maintains a large germplasm collection of maize consist-
ing of 8,000 entries.
Centro loternacional de Agriculfnra Tropical (Cf AT)
Cl AT was established in 1967 by the joint efforts of Ford and
Rockefeller Foundations. The primary responsibility ofCIAT is
hi
International Institutes for Crop Improvement 593
the improvement of cassava and beans. It also looks after maize and
rice improvement in collaboration with CIMMYT and IRRI, respec-
tively. The institute also carries out research work on pasture
management and farming systems. The research work of the insti-
tute is basically directed at solving the problems of Latin
America. CIAT has a collection of 10,000 entries in beans,,
and is fast building up collections of forage grasses and pasture
legumes.
International Institute of Tropical Agriculture (IITA)
This institute was established by the collaborative 'efforts of
Ford and Rockefeller Foundations in 1968. The.primary concern of
IITA is the improvement of grain legumes, and root and tuber crops-
It is also engaged in the improvement of maize and rice in collabo-
ration with CIMMYT and IRRI, respectively. Research work m
cropping systems is an important aspect of the research activities of
the institute. The institute is basically concerned with problems of
the African continent; therefore its research activities -have an African
emphasis. IITA is fast building up germphssm collections of 'grain
legumes, particularly of cowpeas.
West African Rice Development Association (WARDA)
WARDA was established in 1971 by the joint efforts of West
African Governments. The Institute looks after rice improvement
in collaboration with IRRI and IITA. It also promotes regional co-
operative research in rice in the West African countries. The institute
is primarily concerned with the development of rice genotypes suited
to African conditions. -'f-.r . ]'■ s'
Centro International de Papa (CO?)
The International Centre for potato was established in 1971 by
CGIAR. The institute is responsible for the improvement of potato;
particularly for tropical countries. The Institute is also looking after
research work on other aspects of potato production. A world
collection of potato germplasm is being built up at CIP. There are
already 11,000 entries in the germplasm collection ; the number of
entries is increasing at a fast rate.
luternational Crops Research Institute for The Semi-Arid; Tropics
(ICRISAT)
The ICRISAT was initiated in 1972 by CGIAR to increase
the agricultural production in the semiarid tropics. The institute con-
centrates on the improvement .work on jowar, bajra, peanuts, pigeon-
pea and chickpea. Research work on dryland farming systems is an
important aspect of the research activities of me institute. In addi-
tion, a programme to exploit the Rhizahium legume sybiosis in
chickpea, pigeonpea and groundnut is in progress. It is hoped that
this would reduce the requirement of nitrogenous fertilizers in the
semiarid agriculture. ICRISAT is building up a large collection
nf cereals and grain legumes.
etc., e.g„ 9
|ndemic areas), 'bajra
icos 44 etc. in India. ~ ” s^uuumit ' variety
11 is -
I f PGR Was establ ished in 1974 by C qjTJ I8PGR)
Sra? S °T“ fM Sd f SSTm ^ c °"“™iS,"f
•« fs'c* 8 "’ “*S«e
<*■. rice collection aflRRi and il,n ° P ‘ S ^ cific lo ^l pro £ ral “?'
genetic resources trp^d ’ and a so Provides information ® ram ® es *
ditiop far the support from XBPGR r 1 ^? erm Ptesm is a precon
SoT4eSr In ?>fi
j ^ se priority areas for concerted effnrt f ^ as begun to
““ r °° M ««I» » •« of .he .rea“S.v C ° nS ' rVa,i ' > ” »f “»b£
3Zf, PnM, ‘? ar ® f « concerted effort r
fe k irr e s f £S^£rp^°?s s i“
animals of economic importance d deveIo Pment of other
JSBS- C “ re H „ cb „ Drf a
conditions of ? ^ lt,es of £h « institute a« WS®* of
o»"®0".. ,iS MKtorr “- >**'»■>,
Sanctions of Internationa! Institutes
i^tutes is to i n .
'• ^fepetic improvement of the crons samDlanse d as under.
&2— " si!I ™ *-
SSSffiJaSS?" ° f Smap, “" or the concerned
International Institutes for Crop Improvement
595
3. To conduct research on fanning systems for an efficient use of
the available resources.
4. To determine the appropriate technology suitable for the needs
and the resources of the region. The emphasis is to evolve
improved practices from the existing ones so that the local
farmers are able to adopt the new improved technology with
as little difficulty and expenditure as possible,
5. Extension activities to popularise the new technology so that
the cultivator is able to adopt them.
Of these activities, the genetic improvement of crops and the
germplasm conservation are of our immediate interest. Other acti-
vities, though important, are beyond the scope of this book. Many
of these institutes, e.g. y IRRI, ICRISAT, CIMMYT etc., have the
responsibility for breeding crop varieties for several tropical count-
ries. The soil, climate, agricultural practices, prevalent diseases and
consumer preferences would vary to a great deal from one country to
another, and often within a single country. Development of varieties
suited to these varied conditions poes a problem which the breeders
have never faced before. The breeders have attempted to solve this
problem In the following manner.
1 # The first step consists of the identification of the possible envi-
ronments for which the varieties have to be developed. In
each environment a location is selected for the evaluation of
breeding materials and varieties. ■
2. The second step involves making of a large, number of crosses
between parents with very wide genetic base. This is done at a
very large scale. It is hoped that segregating materials from
promising crosses would perform well in one or the other en-
vironment*
3. Finally, the segregating materials from the promising crosses
are tested in the various environments at different locations. A
number of lines suited to those particular conditions are identi-
fied and selected.
Emphasis is laid on testing the breeding material under poor
and good management conditions so that genotypes suitable for
these conditions are identified. The promising lines are freely avail-
able to all the countries for direct release as varieties or for use in
local programmes on crop improvement. The international institutes
no more evolve varieties and promote them. This is in response to
the criticisms levelled against them in connection with the promotion
of semidwarf wheat and rice varieties by IRRI and CIMMYT,
.The role of international institutes in genetic conservation can-
not be overemphasised. Genetic conservation has been the responsi-
bility of local organisations. Due to a lack of coordination, there has
been an 'inevitable, duplication of efforts resulting in a wastage uf ' ■
time, energy and funds. This has also limited the extent to which
596
rw
n S : Principles and Methods
variability in crop species could be collected and „
tion, there were bound to be restrictions on or c f nserved - In addi-
to political and other reasons. The internati<SaSff^ eXChange due
the,r germpiasm collection activities with varlim ^ coordin ate
organisations. This permits a greater efficient £ ** nationai
tion efforts. IRRI has established a comouSriJl! S ® tlC , con s<*va.
for rice which makes it much easier to locate and baaJc
lines from the bank. Free exchange of geneS mL?, the desired
maximum utilization in crop improveiern pJoS * e ° SUres tlleir
Some Contributions of Intarnational Institutes
• *j. ^ te ^ na£ ional institutes have contributed tn
in the agricultural development of the third w n ,M & ® r *? t ex£ ent
increased agricultural production in India if * ouatnes - The
development of semidwarf wheat and rice varieties® rL d “ e to tbe
originated at CIMMYT and IRRj from whfffff Tbes . e varf eties
m India. Lerma Rojo and Sonora 64 wheats f® re “Adduced
ions from CIMMYT. Kalyan Sona and dlrect mtroduct-
segregating generation materials introduced from th? 6 Sele< ? ed froin
Smilarly, IR 8, IR 24, IR 28 and IR 36 vlrietWn^® satne Institute,
duced from IRRI, Philippines. Many of semidnr r i cs . were intro-
developed in this country have IR 8 as on! !f ® varieties
Although the contributions of the international in-ftit ? 611 " pareats;
known in the case of rice and wheat iinorovemfi- la 5* tutcs a « more
m the improvement of other crops are also comiderable C ° ntnbUtIOnS
called 1 ^ »
dwarf wheat and rice varieties. These semidwfr® 0p! ? eat of sem i*
occupied very large areas {approximatelfm d rffr f v f r,etles rapidly
case) in the tropical countries, paSarlv £ °f hec l ares in each.
Philippines There was a considerable Lcrease inti’ Pa . kistai! aad
these crops in the countries concerned This ; he P rodu «ion of
hopes of increase in prosperity and cmtiaol ff- 6 rais ? d faIse
production. Introduction of these varieties leir® in a S n culturaI
socioeconomic problems in these countries whL * 8 n ““ 0er of
below. “ ames are summarised
1 .
2 .
The quality of these varieties was n . ,
example, thered colour of the Mexican whS? b ^ p t° ple ' For
quality of IR 8 rice were not liked K„® ts , an . d tie cooling
these defects have since been acceptabfv^remn* *3 £ nd i! a ' ® ut
work within this country and a numberof « ^ bfee<3in S
are now very popular with the cultivators f Semidwarf va neties
These varieties were susceptible to certain di^
caused considerable loss to the cSSK dise c ases which often
IR 8 to bacterial leaf blight is a f i° rS - Su . sc eptibility of
farmer was often the greatest sufwflf 5n p03at > The poor
afford ^^.im.lyplan.prltmlonSK. JV°" id ”«
.la other hand, generally did no. snff^at 'LSTb^
International Institutes for Crop Improvement
597
¥
he could adopt costly plant protection measures to protect the
crop against diseases and pests.
3. Single pureline varieties came to occupy large areas which in-
creased the 'chances of epidemic development. Luckily, there
has been no serious epidemic so far, although local losses did
occur due to diseases. But this situation was very risky and
has now been corrected to some extent.
4. The most serious effect of the green revolution was the increase
in the gap between the rich and the' poor farmers. Increase
in income led to more mechanisation and an increase in un-
employment. The rich farmers became richer and bought the
land from poor farmers. This caused a considerable increase
in the number of landless labourers.
Thus the green revolution appeals to have failed in the sense
that it could not fulfil the rosy promises that it once held out. But it
has succeeded to the extent that it increased the agricultural produc-
tion in several countries making ■ them almost self-sufficient in food-
grain production. In addition, it has brought into sharp focus the
inadequacies of a hasty “green revolution*’ in the third world. This
has emphasised a more cautious and well planned programme for
agricultural development that would create the minimum of socio-
economic problems.
SUMMARY
There are 11 international organisations for agricultural research. Nine
of these organisations are engaged in crop improvement activities. Some of
them are highly specialised and look after only one crop, e.g. t XRRi for rice
and CXP for potato. But most of them are involved with many crops. The
chief objective of these institutions is to increase agricultural production in
tropical countries. Their activities include crop improvement, germplasm con-
servation, to conduct research on farming systems, to evolve technology suited
for the needs and the resources of the region and extension education to assist
the farmers to adopt the new technology. These institutes have contributed
considerably to the Increase in the agricultural production in the tropical coun-
tries. This contribution is particularly clear in the case of wheat and rice. The
new dwarf varieties of these two crops were responsible for the so called *green
revolution’ that made India self-sufficient in foodgrains. But green revolution
was associated with many agricultural and socioeconomic problems. This
experience has made it abundantly clear that the technology to be used by the
1 farmers in these countries should be suitable for their needs and resources, f,e„
■ it should be developed from the technology existing in the region and not
■Imported from a totally different situation.
QUESTIONS
3. . List the various international agencies and Institutes concerned with the
Improvement of agricultural production. Briefly describe the objectives
and the functions of any two of them.
598
3.
Plant Breeding i Principles and Methods
Discuse the functions of international institutes. Discuss the chief con
tributions made by them in solving the problem of low agricultural nro
ductivity in tropics and subtropics. 6 ra « pro-
Write the full names of the following : CGIAR, CJAT, CIMMYT rru
ICE,SAT - ,,TA ’ ,LCA - fSarefi
Suggested Farther Reading
F ^A*Jif&om52 T :698Tor VOlUti ’ 0n '' Seacmhas of Problems. Amer. J.
Sen, B. 1974. Green Revolution in India, a Perspective. Wiley, New Delhi.
SlMM °^d New York.' Principles of Crop Improvement. Longman, London
Glossary
Adaptation. If is the process by •which organisms become more suited
to survive and function in a given environment. It also refers to
the result of this process.
Addition Line. An addition line has one pair of chromosomes from
another variety or species in addition to the normal somatic
chromosome complement of the species.
Agronomic Trials Evaluation of new strains for release as a variety ;
an entry showing superior performance in the first year of URT
is included in trials designed to determine the optimum date of
sowing and irrigation level ; under the respective All India
Coordinated Crop Improvement Projects.
Alien-Addition Line. It has one pair of chromosomes from a related
wild species in addition to the normal somatic chromosome
complement (2 «) of the species; : "
Allele. Alleles are alternative forms of the same gene, and are located
at the same point (locus) in homologous chromosomes.
Allogamy. In allogamy, pollen grains from flowers of one plant
pollinate the flowers of other plants (Syn.,' cross-pollination).
Allopolyploid. A polyploid containing two or more different
genomes. ; //T /v ///"t
Amphidiploid. It has two copies of each of the two or more different
genomes present. Thus an amphidiploid has the somatic chro-
mosome complement of two or more diploid species.
Aneuploid. An aneuploid organism has a chromosome number
that is not an exact multiple of the basic chromosome number
(x).
Anther Culture. Culture of anthers (or pollen grains) on a suitable
medium for the production of callus and/or haploid plants,
Anthesis. The first opening of a flower. i >
Antibiosis. The adverse effect of feeding on a resistant host plant
on the development and reproduction of the inseet pest.
Apogamy. Development of embryo from synergids or antipodal cells
without fertilization ; a form of apomixis.
Apomixis. Development of embryo (and seed) without fertili-
zation. ■ ' yryy/
600
Plant Breeding : Principles and Methods
Apospory. A form of apomixis in which the embryo sac develops
from a vegetative cell of the ovule.
Artificial Selection. Selection by man.
Asexual Reproduction. Asexual reproduction does not involve the
fusion of male and female gametes.
Autogamy. Pollen grains of a Sower pollinate the same flower (Syn,
self-pollination).
Autopolyploid. A polyploid that has more than two copies of the
same genome.
Avoidance. Escaping of a susceptible host plant from damage due
to an insect pest, usually, as a result of the host plants being at
a much less susceptible developmental stage when the pest popu-
lation is at its peak.
Bi and B t . Backcrosses of Fi to Pj (first parent), and P t (second
parent), respectively, of the hybrid.
Backcross. A cross between a hybrid 1 and one of its parents. Also-to
inake such a cross.
Backcross Breeding. A breeding method based on repeated back-
crossing of the Fi and the subsequent generations to the recurrent
parent,
BC U BC a , BG» etc. Progeny from FiX recurrent parent, BC r X re-
current parent, BCa x recurrent parent etc., respectively.
Basic Number. The haploid, chromosome’ number of' the aneestra!
diploid species of a polyploid. Represented, by. x. Also. hapioidi
chromosome number of diploid species, x chromosomes constitute
a genome.
Basic Seed. Seed produced by mass selection (with progeny, test), in
a pureline variety or clone ; source of breeder seed.
Biometry. It consists of the application of statistical procedures to
biology.
Biotype. Strains of a species of pathogen, particularly an insect pest,
differing in their ability to attack different varieties of the. same;
host species (Syn. physiological races).
Biparental Cross Analysis. A design for genetic analysis based on
progenies derived from the matings involving a random sample,
of F* or later generation plants from a cross between two pure-
lines ; the four designs based on such progenies are NCDI.
NCDII, NCDIII and TTC. F 8 **
Bivalent. Structure formed by pairing of two homologous chromo-
somes during meiosis. •
Glossary
Breeder Seed. Seed produced by tbe breeder or the institute which-
developed the variety. Source of foundation seed.
Bridging Species. A species used in gene transfer from one species
to another sexually incompatible species ; bridging species is
compatible with both the donor and recipient species ; it may be
a natural or synthetic species.
Bud Mutation. Mutation in somatic tissues, usually affecting an
axillary bad.
Bud' Selection. A fbrm ofdonal selection io which, mutant bad® am
selected.
Bulk Method. In this methods, Fa and the subsequent generation*
are grown in balk, usually without artificial selection. In the
end, pureline varieties are developed through individual
selection.
Certified Seed. Seed produced from foundation, registered or
certified seed. Its purity is certified by a seed certification agency,
and is usually used for commercial crap production.
Centres of Diversity. Areas where cultivated plant species and/or
their wild relatives show much greater variation than anywhere,
in tbe rest of the world.
Centres of Origin. Areas where- cultivated plant species are supposed
to have originated. Baaed, on. centres of diversity.
Centromere.. The localised region of a chromosome where spindle
fibres attach,, and which is responsible for. chromosome move-
ment during cell division.
Character. A morphological, anatomical or physiological feature of
an, organism ; usually a product of the actions of both genotype
and environment.
Chiasma. The point of contact between nonsister chromatids of
homologous chromosomes during and after dipiotene (meiosisX
mera. An individual that is composed of ceils of two or more
genotypes.
omatid. Substructures of chromosomes produced by replication
preexisting chromatids. Each chromosome has two chromatids
hich appear fused at the centromere.
mosorne. Thread like structures present in nucleus which show
istinct change in their morphology during cell division. They
are deeply stained with some dyes, particularly with the Feulgen’s
reagent. Genes are located ; n chromosomes in. a linear order.
Chromosome Manipulation Techniques. Techniques used for promot-
ing gene transfers from chromosomes of a related species into
those of a cultivated species.
450
son
and
Eve
part
the j
ever
tbeM
Plant Breeding : Principles ana Methods -
Chromosome Substitution. The procedure or the *
one pair of chromosonies P of a variety wi?h^ho fi enf Cp aC i n *
vanety or a related species. The latter h aUeJsubrttotSn *
eiXfrtamm,* s __
r, t . mww is a/ie* substitution
c ,rf::i"ri:fr.°: r - ^ ,e -
r . ... .. , , . mere IS no antfiesis.
Clone. Individuals obtained from a sinaJe olant th. «.
reproduction (produced by mitotic celfiiviaiS). througfa asexU8 l
j wv-ii ui vision/#
Clonal Multiplication. Multiplcation through asexual reproduction
^ «*»«• » <<*
C “w1“e °Cm“o! Th a i T ^,"p“« ” f ,‘S ri de,i «»” of a sample
[(s/x)xi00l ^ expressed in per cent.
J! Jsffi M* «”* . 0.0 « . fw
another. ^ °%®genes, from one variety to
upon hybridiMtio^wi^other^Sns. 0 produce * upcrior P^gcny
mixture ofseverai genotypef! ilty ° f * genotype to 8Urv >ve in &
Complex Cross. See Convergent Cross.
c ^^MSJsfSs. r™*, ft »" » <■».
Pi or F, generations from ail possibte^CTowl 18 leed * ff om tJl ®
ordering them into a comr>lnT P nrnlo cr< ?? ses . among them, bjr
with a male sterile line and harvMt^’ ° * OWI 5 g *kem *° cross
male sterile plants in each genemS g S< * ds from oniy
“ — OrUVJU*
at numbe^of outstanding strains usuaHv open ' pop “! ation among
ability with each other ® * USUaiJy not tested for combinin|
Contiguous Control. A control nr
adjacent plot j a design for the vai l ety pian ted in an
lines/progenies in which the enn^r!^ 3 ** 0 ^ of • a large num ber of
5th or 7 th row/plot (auJemffTf 7™! I s P^ted in every
the line/progeny means are madt S the^onSol COmparisons of
_ — — 'UA9 ..wxiiroi*
~i¥crg€nt L,F0SS„. A pmoe . '
Convergent c rM . a . . uuriD g tneir evaluation,
complex cross). Cr ° SS mvoiving more th an two parents (Syn.,
»o yean) coSw" mSHte rrapSv e’ain d'^r ( “ !,1,s for
***** CVT - < s »- raT/Unifor^ReSilTriair" * m
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ChroiBf',:
one ' :
Glossary
Correlated Response to Selection . Change in one or more quantitative-
characters due to a selection for another character.
Coupling Phase . Linkage between dominant alleles of two or more
genes.
Cross, Mating of two or more individuals or strains having different
genotypes. Also the product of such a mating.
Cross-Pollination . See , allogamy .
Crossing Over . Exchange of homologous segments between non-
sister chromatids of homologous chromosomes ; takes place
during pachytene.
Cytogenetics* Study of chromosomes in relation to genetics.
Cytoplasmic Inheritance . Transmission of characters through
cytoplasm. It is due to the DNA present in cytoplasmic
organelles.
Cytoplasmic Male Sterility * Male sterility showing cytoplasmic
inheritance.
Cytoplasmic-Genetic Male Sterility . Cytoplasmic male sterility for
which a restorer gene is available/known.
Deficiency . Loss of a part of a chromosome.
Detasselling . Removal of the tassel (the male inflorescence) before
it sheds pollen ; in maize. An easy method of emasculation.
Blallel Cross . Crossing of a number of genotypes in all possible
combinations. .
Diallel Selective Mating Scheme ... A breeding scheme in which'
several selected purelines are mated according to the diallel
scheme; the promising F 2 populations are advanced by self ing '
tor the isolation of purelines to be used as varieties ; simulta-
neously , the selected plants in the Fa generations are intermated
o generate populations which will again be handled as above,
inis process continues.
Dichogamy. Maturities of male and female reproductive organs of a
hermaphrodite flower at different times.
Dihaploid. Haploid derived from tetraploid, i.e,, monoploid for two
distinct genomes. '
Di hybrid. An individual heterozygous for two genes.
Dioecious. Plant species in which unisexual (male and female!
■ flowers occur on different plants. Such species have male and
lemaie plants.
Diploid. An organism having two genomes, with chromosome,
number of 2k.
$04 Plant Breeding : Principles and Methods
Diplospory. A form of apomixis in which the embryo sac develops
from the megaspore.
Biplotene . Stage of meiosis following pachytene in which homo-
logous chromosomes forming a bivalent (or multivalent) begin
to move away from each other. Chiasma is seen during this
stage*
Disease . An abnormal condition produced by an organism.
Disjunction . Separation of chromosomes at anaphase.
Distant Cross . See Cross*
Distant Hybridization . Hybridization between individuals- belong-
ing to two different species of the same genus or of different
genera.
Disomic. An individual with two homologues for each chromosome
of the genome or genomes.
Dockage . Per cent impurity in- a seed sample*
Dominance . Ability of an allele to express itself in the heterozygous
state.
Domestication . The process of bringing a wild species under human
management.
Donor Parent. In backcross breeding j the parent from which one or
few genes are transferred to the recurrent parent (Syn. non-
recurrent parent).
Double Cross. Cross between two single crosses (i.e. 9 between- two
Fjs from two single crosses).
Doubled Haploid * A plant or line obtained by doubling the chromo-
some number of a haploid plant/individual.
Doubled Haploid Technique . A technique in which homozygous
lines are obtained by doubling the chromosome numbers of
haploid plants extracted from heterozygous (usually F x ) plants.
Drift. Random change in gene and genotype frequencies in small
random mating populations.
Duplication . Occurrence of a chromosome segment more than twice
in a diploid chromosome complement. The duplicated segment
may occur in the same or in a different chromosome.
Early Testing. In self-pollinated ' crops, evaluation of the perfor-
mance of early segregating generations (usually, ' Fa or F* bulks)
or F3/F4 individual plant progenies in order to 'assess the' worth ■
ofpurelines that would be isolated from them ; in cross-pollina-
ted crops, the estimation of GCA or SC A of plants/progemes
early in the the processs of inbreeding.
Glossary
Emasculation. Removal of immature anthers (or androecium) from
a hermaphrodite flower.
Embryo Culture. Removal of developing embryo from seed and its
cultivation in vitro.
Embryo Sac. Cell derived from megaspore. It contains egg cell,
synergids, antipodals and polar nuclei (Syn., megagametophyte).
Entry . . A newly developed line/strain included for evaluation in
the multilocation trials of the various All India Coordinated
Crop Improvement Projects.
Environment. Sum total of external conditions which influence an
organism.
Epidemic. Development of a disease from a low infection level to
a high intensity of infection. Artificial epidemics are created by
man by providing the inoculum and the suitable environment
for the disease development and used in disease resistance
tests.
Epiphytotic. See Epidemic.
Epistasis. Interaction between two different genes, that is, expres-
sion of one gene is affected by another gene and vice-versa.
Error Variance. Variance due to factors beyond the control of
experimenter. (It is used as the denominator in F-test).
Escape. More commonly disease escape. Phenomenon of susceptible
plants avoiding disease attack. Also plants showing escape.
Euploid. Individual whose chromosome number is an exact multiple
of the basic number.
Evaluation. The process of assessing the performance (yield, quality,
disease and insect resistance, etc.) of newly developed strains
of a crop through appropriate multilocation trials.
Excitation. Movement of an electron to an outer orbit of a higher
energy level.
Exotic. A yariety or species introduced from a foreign country.
Exploration. A trip for collection of germplasm of cultivated and.
related wild species (fiyn. Expedition).
Expressivity. Ability of a gene to express itself uniformly m all the
individuals that carry it in the appropriate genotype.
Extreme Homozygote. A homozygote isolated from a hybrid,
usually produced by crossing two or more parents, that has
either all the positive or negative alkies for a trait for which
the hybrid was heterozygous.
450
wou
mou
506
Plant Breeding : Principles and Methods
Mon
in p
Mon
and \
Locm
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*rom<
a one
I. A
intermating among
and
F v The progeny obtained by crossing two different genotypes. The
first generation from a cross.
F s . Progeny obtained by seif- fertilization of or
Fi individuals.
F 3 , F t ere. SeliVd progeny of F ; >, F 4 etc., respectively.
Factor, See Gene.
Family. A group of individuals sharing a common parent/ancestor.
Fertility. Ability to produce viable progeny.
Fertilization. Union of male and female gametes in sexual reproduc-
tion.
Foundation Seed. Seed produced from breeder seed. It is the source
of registered and certified seeds.
Free Radical. Electrically neutral molecules with an unpaired elec-
tron in the outer orbit. Free radicals are highly reactive.
Gamete. A specialised cel! produced by gametogenesis that partici-
pates in fertilization. Gametes are produced through meiosis.
Gametic Chromosome Number. Chromosome number present in
gametes of a species.
Gametogenesis. Production of gametes from spores.
Gene. Functionally, gene is unit of inheritance j one or more genes
control the expression of a character, and one gene codes for a
polypeptide. Structurally, gene is a segment of DNA which codes
for one polypeptide, ribosomal or transfer RNA.
Gene Bank. Large collection of germplasm representing materials
from various parts of the world (Syn., World Collection).
Gene Frequency. The proportion of an allele present in a population,
represented by p and q.
Gene Interaction. Modification of expression of a gene by one or
more nonallelic oligogenes.
Gene Pool. Sum total of genes present in a Mendelian/panmiciic
(random mating) population.
Gene Sanctuary. An area within the centre of diversity protected fro*
human interference.
Genetic Advance (under selection). Improvement in the performance
of selected lines over the original population.
Genetie Emasculation. Use of genetic factors to make the male
gamete nonfunctional in self and or cross-pollination.'
Genetic Equilibrium. In a random mating population ; the stage in
which genotype frequencies do not change from one, generation
to another. It has reference to one or more genes.
Glossary
Genetic Erosion. Gradual disappearance of various forms of a
cultivated species and of its wild relatives.
Genetic Load. The sum total of deleterious (harmful) alleles present
in a random mating or Mendelian population.
Genetic Purity of Seed. Freedom of seed from seeds of weeds, other
crops and other varieties of the same crop.
Genetic Variance. Variance due to genotypes of different plants
or strains. It has additive, dominance and epistatic
components.
Genetic Vulnerability. The susceptibility of most of the cultivated
varieties of a crop species to a disease, insect pest, or some other
stress due to similarities in their genotypes 3 usually, due tr
the presence of one (generally more) common parents) in thei ;
ancestry.
Genetics. A study of the mechanism of transmission of
characters from parents to offspring, origin of variation and gene
action. D
General Combining Ability. Average performance of a strain in a
series of cross combinations. Estimated from the performance of
‘Fx’s from the crosses.
Genevation Mean Analysis. The estimation of additive, dominance
and epistatic gene effects from the means of certain specific
608 Plant Breeding ; Principles and- Methods
Heterocaryosis. In fungi, a condition in which a hypha has nuclei of
two different genotypes.
Heteroploid. An individual with a chromosome number other than
the normal diploid (2^) number.
Heterosis. Superiority of over the parents (or even inferiority to
both the parents, e.g., earliness). {Syn., Hybrid Vigour).
Eeterastyly. Occurrence of styles and stamfeas of different lengths
in Sowers from different plants of a siiigfte spteies.
Heterozygous. As. indh&doe! - hawing -ikmeAs e&Mes «f &
Scmoeoiogms Chromosomes. ¥art»% hcmblogeos chromosomes
generality show reduced or lade bfpairinjj.
Homologous Chromosomes. Chromosomes identical with estch other-
in gene content and moiphofbgy ; two homofosousohroxn^otites
pair to form one bivalent dtiriag raeiosis.
Homogeneous. Consisting of individuals of the same genotype or
phenotype. * r
Homozygous. An individual having two cat store identical aiv.fr, 0 £
the same gene.
Horizontal Resistance. Resistance govomed fey
patfeotype nonspecific.
rn^m^s ma is
EM. The organism attached by a pathogen.
Hybrid. Progeny from hybridisation between two or more
Hybrid Subsicnce. New isozyme present in ’the hybrid which is
different from those present in the parents. 7 wcica is
Hybrid Varieties. A hybrid variety » F* gesomtionfrem aiSross
between two different (strains. ^ross
Hybrid Vigour. See Heterosis.
Hybridization. Mating of two different strain*.
h, hr h etc. Generations after one, two, three etc.. teieectivefr
generations of inbreeding. * respectively,.
Identification of A Variety. Recommendation by the attmtal worir
shop of the concerned All India Crop Improvement Prelect for
the release of a strain or an entry as a new variety.
^Xease^^"' €omplete absence of symptoms of a
Improved Seed. Seed of an improved variety having a very hiak
genetic and physical purity, and germination. . 8
Glossary
609
In vitro Techniques, The technique of culturing plant ceils and
organs on artificial media in vitro.
Inbred Line. A nearly homozygous line developed by continued in-
breeding, usually selling, accompanied by selection.
Inbreeding. Mating of individuals more closely related by ancestry
than would be expected under random mating,.
Inbreeding Depression. Loss in vigour due to inbreeding.
Inbred-Variety Cross. Cross between an inbred and an open-polli-
nated variety.
Infestation. The attack by an insect pest on the. host; plant.
Initial Evaluation Trial. The preliminary evaluation of a relatively
large number of newly developed strains in the multilocation
trials of the concerned All? India Coordinated Crop Improvement
Project; the purpose is to reduce the number of strains to a
manageable level for a relatively more extensive evaluation in
TJRT or CVT ; ommitted m crops? where the number of entries is
not large.
Insect pest. An insect species which causes (damage to a crop
species ; they may be. tissue feeders or sucking pests.
Insect Resistance. Th.e property of a host variety to be attacked
by an insect pest to a noticeably smaller degree than some other
varieties of the same host speeds.
Jntergenotjpic Competition. Competition between or among plants
* of different genotypes ; important .in the segregating generations
varietal mixtures, etc. *
Interplant Competition. Competition among plants of the same
genotype ; important in the stands of purelines, single cross
hybrids, clones etc.
Inert Matter. In a seed sample; all nonliving matter, disease and
insect infested seeds and broken (damaged) seeds.
Interspecific Hybridization. Hybridization- between plants belonging
to two species of the same genus.
Introduction, Plant. Taking a variety or a species into an area where
it was not grown before ; even within a country.
Introgression. Transfer of a few genes from one species into the full
diploid chromosome complement of another species.
Inversion. A segment of a chromosome is rotated by 180° so that the
gene order on that segment is reversed.
Ion An atom that has lost or gained an electron in comparison to
the number of electrons normally present.
Ionization. Loss or grain of an electron by an atom.
450
wou 610
men
Plant Breeding : Principles and Methods
It is the arithmetic
set of observations. [(ZX)/N\
Mon-
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Irradiation. Exposure of biological materials' to radiative
particularly to mutagenic radiations. • 9 more
Isogenic Lines. Lines identical in genotype except for one gene.
Isolation. Separation of two or more plants, strains or pooulatinn* *
bS amo “ s ,h8m - u,ra "- v “ hie ™ d ->y ■£££ S/m
Kinetochore . See Centromere .
^ e o,te ae “° wb “ h *« w *
Lethal Gene . Usually recessive ; lethal senevkilf mpI, * a
individual that carries them in the appropriate genotype eVMy
Line. A group of individuals having common parents or ancestors.
Line X Tester Analysis. The estimation of GCA and SPA -«•,> *
and variances from progenies developed by matins eSh
several lines to each of the same selected te£
SrtSsr°“ var " n “ 5 are estimattd
Tendency of two or more genes to be inherited together.
the position^ of varions gene^prOsent iifthe^°f 0S d meS ? how «»g
their linkage relationship. P them as de£ ermined by
L^The position at which a gene is located in a chromosome.
1 third etc. genw^ons/^Sely^derfved byfeffi 1 &St ’ second ’
reproduction. y ’ aenved »y selfing or asexual
Maintainer Line. Line used for ms»int a ;„;„,.
J? 1 ■* **“ *• , JSSiSSfS JS
Ab “° Ce ° f f “ ni,,i0Ml **m<M (pollen gcai „ s
W-M- - maintained
o=,r ; a, «, ata^^eS STSWSSS 1^“
« ssas- 0 ^ - ^ is
WMCl1 ‘ ndividuals or »"« “re
Glossary
Megagametogenesis. Production of the female gametophyte (embryo
sac) from megaspofe (through mitosis).
Megasporogenesis. Production of the female spore (megaspore) from
megaspore mother cell (through meiosis).
Meiosis. Two cell divisions, occurring one after another, after only
one DNA replication ; shows chromosome pairing and chromo-
some separation in the first division and chromatid separation
in the second. Leads to the production of haploid cells (spores/
gametes).
Meristem Culture. Cultivation of apical meristems, particularly shoot
apical meristem, for the production of shoots and plantlets.
Mericloning. Vegetative multiplication through meristem culture.
Metaphase. A stage during cell division during which the chromo-
somes lie at the metaphase plate (equatorial plate).
Metaxenia. Effect of pollen grains on maternal tissues of fruits.
Microcentre. Areas within centres of origin that show greater diversity
than the remaining centre of origin.
Microganteto, genesis. Production of the male gametophyte from
microspore (through mitosis).
Microsporogenesis. Production of pollen grains (microspores) from
microspore mother cell (through meiosis).
Migration. Movement of individuals from one population into
another.
Mitosis. Cell division in which the two chromatids of each chro-
mosome move to the opposite poles producing two In daughter
nuclei. Every mitotic division follows DNA replication (compare
with meiosis).
Modifying Genes. Genes with no phenotypic effect of their
own, but change the expression of some oligogene. They have
small, cumulative effect producing a continuous range of
phenotypes.
Monoecious. A plant having both male and female flowers.
Monohybrid. An individual heterozygous for one gene.
Monoploid. An individual with the basic chromosome number ( x ),
i.e., with one genome.
Monosontic. An individual with one chromosome less than the
somatic chromosome number (2 n— 1).
Multiline Varieties. Mixtures of several similiar purelines having
different genes for disease resistance.
450
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612 Plant Breeding ; Principles and Methods
Multilocation Trials, Trials . conducted at several locations under
the various All India coordinated crop Improvement Projects*
Multiple Alleles . More than two alternative forms of a single
gene.
Multivalent. The structure formed by the association of more than
two homologous chromosomes as a consequence of synapsis
during meiosis.
Mutagen . A chemical or physical agent that induces mutation.
Mutagenesis . Induction of mutations with the aid of mutagens'.
fUril!
Mutations. A sudden heritable change in a characteristic of an orga-
nism. Macromutation produces a sufficiently large change to be
easily identifiable, while micromutations produce small changes
(usually in quantitative traits) and are studied in terms of mean
and variance.
Mutation Breeding. Breeding method utilizing variation created by
mutagenesis.
n. Gametic chromosome number of a species.
In. Somatic chromosome number. The chromosome number present
in somatic cells of a species.
National Evaluation Trial (NET). In oilseeds ; the plot size is three
times the size of .that in CVT ; the entries are identified for
prerelease multiplication only after NET.
National Trial. In some crops,- e.g., wheat ; the topmost entry in
CVT or URT of each, zone is evaluated in an IET throughout
the country to assess if an entry developed in one zone may
perform well enough for release as a variety in another zone of
the country.
Natural Selection. Selection, due to natural forces, i.e., environ-
ment.
Noblisaiion of Sugarcane. latrogession of genes and chromosomes
from noble canes and wild Saccharum species into Indian canes
(S. barberi).
Nonpreference. A host variety being unsuitable or unattractive to
a n insect pest for colonization, opposition or both.
Nonrecurrent Parent. In backcross method ; the parent from which
one or few genes are transferred to the recurrent parent (Syn.,
donor parent).
North Carolina Design / (NCDI). A mating design used for genetic
analysis ; four random plants are mated as females to one ran-
dom plant used as male to generate one male group ; four male
groups make a set and several se& may be generated in one
experiment; applicable to the segregating generations (self-
pollinated species) or random mating population.
irvivi
itica
Glossary
613
North Carolina Design H(NCDH). Equal number of male and
female plants selected randomly and mated in a diallel fashion to
yield m xf number of crosses (m=» number of males, /—number
of females) ; applicable to the segregating generations (self-
polliaated species) or random mating populations.
North Carolina Design HI ( NCD HI). Several random plants selected
from an Fa are bickcrossed to the two parents of the concerned
F 2 generation.
Notification of a Variety. A notification from the Director, High
Yielding Varieties, Ministry of Agriculture and Irrigation,
Government of India, to the .concerned authorities for the seed
multiplication of a variety just, released fqr cultivation by the
central or a state Variety Release committee. ■'
Nucleus -Seed. See Basie Seed,
NuUisomic. An individual having a. pair of homologous chromosomes
less than the somatic chromosome number of the species
(2n-2).
Off-season Nursery. Grp. wing, the breeding materials during the off-
season (a season in which the crop is not grown in the concerned
locality) in a location suitable for the purpose ; ■ generally to
advance the generation.
Oligogenes. Genes having, large individual effects producing distinct
phenotypes.
Open-Pollination. In cross-pollinated species, pollination occurring
naturally without restriction.
Outer oss. Natural cross' between two different genotypes.
P lt P‘i, P-i etc. First,- second, third etc. parents; respectively,
used for producing a hybrid or a series of hybrids. Abo the
first, second, third etc. ^ generations of parents obtained by
selling. '
Pachytene. Stage in meiosis ; when pairing between homologous
chromosomes is complete giving a single-stranded appearance to
the bivalent. Crossing over occurs daring this; stage.
Panmixis. Random mating without restriction.
Parameter. A numerical quantity which describes some characteristic
of a population.
Path Analysis. A standardised partial regression coefficient analysis
which splits the various correlation coefficients ‘into the -measures
of direct and indirect effects of a set of independent variables
on a dependent variable (usually, yield).
Parthenogenesis. Egg cell of an embryo sac derived from megaspore
develops into an.embryo without fertilization.
450
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614 plant Breeding : Principles and Methods
Pathogen. An organism producing a disease.
Pathotype. -Strain of a pathogen virulent toward a specific resistance
gene of the host. * distance
Partial Diallel Analysis. A diallel analysis based on a specified
sample Oi crosses from among the all possible crosses (diallel
scheme) among a number of parental lines. v
P edigree . A record of ancestry.
Pedigree Method. A method of breeding in which individual nlam*
are selected in F 2 and subsequent generations and a pedigree
*gei
Jroxr
low
Dtild
rviv»
idea,
'ram
5 rec
iefa
‘OOS
i ad
omi
>ne :
record is maintained. The pedigree record may be used as a basis
for selection in later generations.
Penetrance. Ability of a gene to express itself in the individuals
carrying that gene in the appropriate genotype. Penetrance may
be complete or incomplete. ^
Phenotype External appearance of an individual with reference to a
single character or a number of characters.
° f s “ dffrom *”"• tacM..
Physiological Races. Strains of a pathogen species that differ in their
SK t0at !\ ckdiffe ^ v&Tkties 0{ & host species^ “SE
pathotype, it has no reference to the resistance genes.
Plant Breeding. The branch of biology concerned with changing the
genotype of p tals „ they bVcotne more “ “*
."ifcSir ° f a siD8,e e “ c affMto8 ‘™ »'
Pollen Culture. Cultivation of pollen grains in vitro for nrodueino
haploid plants (generally referred to as anther culture). P 2
Pollination. It consists of pollen grains renehin
Glossary 615
Population Cross. A cross between two open-pollinated varieties (Syn.,
varietal cross).
Population Improvement. Improvement of random mating popula-
tions through a scheme of selection with or without progeny
test. It is essential to keep inbreeding to a low level.
Prepotency. Ability of an individual to produce progeny which are
similar to each other and to itself.
Probability. The proportion of times an event may be expected to
occur if an infinitely large number of trials are made.
Progeny Test. Evaluation of the genotypic value of an individual
on the basis of the performance of its progeny.
Prapagule. Plant part used for propagation.
Protandry. Dehiscence of anthers before the stigma of the flower has
become receptive.
Proiogyny. Maturition of stigma (becoming receptive) before the
dehiscence of anthers of that flower.
Pureline. Progeny of a single homozygous self-pollinated plant.
Pureline Selection. Isolation of pureliaes from a mixture nf purelines
{Syn., individual plant selection).
Quadriallel Analysis. Analysis based on double crosses obtained by
crossing n homozygous lines in a diallel fashion, and then crossing
the Fi’s so generated according to a diallel scheme with the
restriction that in any double cross a homozygous line must not
occur more than once as a parent.
Quadrivalent. Structure produced by pairing among four homo-
logous chromosomes during meiosis.
Qualitative Character. Character showing distinct classes and little
or no effect of environment.
Quantitative Character. Character showing continuous variation and
considerable effect of environment.
Quarantine. Isolation of an organism for observation on weeds,
diseases and pests and for preventing their spread.
Random. An event whose occurrence is determined solely by chance
and there is no discrimination.
Random Drift. Set Drift.
Random Mating. A system of mating in which an Individual has
equal chance of mating with every other individual of the
same population.
Range. The difference between the highest and the lowest values of
observations in a sample.
450
WOtl '
mon
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in p
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^ breeding : Principles and Methods’-
RealValut! of Seed. The percentage (by weight) of the seed samole
which would produce seedlings of the variety under certffi!
canon.
Recessive. An allele unable to express itself in heterozygous . state.
Reciprocal Cross. Cross in which the line previously used as male
is used as a female, while that previously used as female is used
as male*
Recipient Parent. In feackcross breeding, fitc parent to which on a-
or few genes from the donor parent are transferred (Svn
recurrent parent).
Recombination. Production of new combinations of genes as *
result of independent segregation of nonalleles or crossing over
between linked genes ; usually the latter. 8
Ol S “3 Sf '• overco™,
Sample A set of random observations taken from a population.
Recurrent Parent. See Recipient Parent.
Recurrent Selection In cross-pollinated populations : schemes of
selection (on the basis of phenotype or progeny test) followed
by interring (in all 'combinations) of tht relected plant? ot
their selfed progeny to produce the population for the next
cycle of selection, More than one cycle of selection practised
Registered seed. Progeny of foundation seed.
Release of A Variety. The recommendation by the Central ora
tate variety Release Committee of an improved strain of a <’- 0 ti
country 1 COmmercia! cu3tivation in the whole or a part of thf
Production of a new generation of 'individuals fnro
geny) by sexual or asexual means! .muiviauais ^ r °-
Repulsion Phase. Linkage between the dominant allele n f n ,»
with the recessive allele of another gene. * 1 * ae gsas
Roguing. Removal of off-type plants of the same species.
Resistance. Ability of a host strain tn
production of disease symptoms by a Shown P ? Vent tfae
for resistance. y a P atbo 8 en • <toe to a gene
ReS hT C LZ\oped tie f TbilitJ h or adaption To iSg f* ? hicb
variety of the host. *uapiauon to infest a resistant
Glossary 01 /
Sampling Error . Deviation of a sample' value from that of the
■ population due to the small size of the sample.
Seed. Fart of a plant used for raising a seed crop or a commercial
crop*
Seed Test. A series of tests on purity* moisture content and germi-
nation of a seed sample to determine its quality*
Seed Certification. Seed certification consists of field inspection
and seed tests to ensure genetic and physical purity’ and good
germination of seed lots, and issuance of a certificate to that
effect.
Segregating Generations . The F g and the subsequent generations
obtained through continued selling of a hybrid between two or
more strains.
Segregation * Separation of alleles at the time of gamete formation
so that each gamete receives* only one of the two alleles of a
gene.
Selection . Differential reproduction rates of different genotypes.
Self* Fertility . Ability to setfseeds on self pollination,
Self-Fertilization , Union of male and female gametes from the same
individual.
Self-Incompatibility . Lack of seed set on self-pollination.
Self-pollination . See autogamy *
» ■
Short-Term Inbred. A line derived by one or a few generations o l
lab reeding. Such a line is not homozygous or even nearly
homozygous.
Sits. Individuals having both the parents common, but derived from
different gametes /.<?., they are not identical twins, Half-sibs art
individuals having one common parent.
Sib Mating . Mating between sibs.
Simple Correlation Coefficient * A measure of the relationship bet-
ween two variables.
Single Cross. A cross between two lines, usually inbred lines.
Single Seed Descent Method . A method of advancing the generat-
ions ; one seed from each F 2 plant of an F» and the subsequent
. generations is bulked to raise the next generation/
Species . A unit of taxonomic classification. Members of a species
afe more like each other than those from different species, and
do not show barriers in sexual reproduction (except, of' course,
in cases of self-incompatibility and male sterility etc);
'Specific Combining Ability, Deviation in performance of a cross
combination from that predicted on the basis of the general
combining abilities of the^ parents; tnvolyed m the cross.
450
won.
non
Hm
n p
tffOfl
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ocm
ions
•tryi ,
** 8 & lani Breeding : Principles and Methods
Sp orogenesis. Production of mega- and micro-spores from meg*
and micro-spore mother cells, respectively (through meiosis).
Stability Analysis. An analysis to estimate the adoptabilitv of »
genotype ; adaptability is the ability of a genotype to produce
mSts ' y Uarr ° W r3Dge ° f phenoty P es ifl ^e different environ.
Standard Deviation. Square root of variance (see variance), it is a
measure of spread (variation) of a sample or population. (vO*)
Standard Error. Standard deviation of a sample of means estimat-
ed from a number of random samples drawn from a sinsle
population. It is estimated as standard deviation divided by
square root of the number of obervations in a sample. ('\Zs*Tl V)
Station Trial. The. trial conducted by a breeder to asses the per-
formance of strains evolved by him as compared to a standard
check ; usually at one location. (Syn., Preliminary Yield Trial).
Stock Seed. The seed produced by a breeder of a strain which has
been identified but has not yet been released. It will be known
as breeder seed once the strain is released and notified.
Strain. A group of individuals similar in phenotype and often in
genotype. A strain is known as a variety when released for
commercial cultivation by a variety release committee.
ias be «
Used in aneuploid analysis. £ty * the Same species -
Sy/to. The parental lines of a synthetic variety.
Sw. lines in all
Syno lines (parenS fin'es) * y ° pea " P ° lhnatl0n amon g «»e
dcrived throu S h ™dom mating from
•W 0 * etc. generations, respectively.
homo,ogous ^romosomes during meiosis,
P-trtu.uiaUy during zygotene stage of Prophase I.
Allopol yP loid - Allopolyploid produced experimentally by
^ "by irn, Va] i K ted - species ’ a variety obtained
c-'-riref-eU fill! ? combinations a number of lines that
latergensraiiorf 4 ** * ° thei; Syai or ’ U10re generally, Syn 2 and
£ vicin' tJd^rl^nh ia dl vision after the chromosomes/
<««* **» **> »=for. .he nuefeal
■Glossary
Test Cross. In genetics, cross between a hybrid and the homozygous
recessive strain. In plant breeding, cross between a plant or line
and a tester (the tester may be an- inbred, hybrid, synthetic or.
open-pollinated variety).
Tetraploid. An individual having four identical or distinct genomes.
Tetrasomic. An individual having one pair of chromosomes in
addition to the normal somatic chromosome complement of tfco
species (2n-f 2).
Three-Way Cross. A cross between a single cross and an inbred.
Tissue Culture. Cultivation of plant cells and tissues in vitro on
artificial media.
Tolerance. Ability of a host to avoid or minimise loss in productivity
although it has been infected by a pathogen.
Top Cross. See Inbred-Variety Cross.
Trait. See Character.
Transgressive Breeding. Breeding methods designed to utilize
transgressive segregation.
Transgressive Segregation. Appearance of individuals in the progeny
from a hybrid which exceed either of the two parents of the
hybrid with respect to one or more characters.
Translocation. Incorporation of a chromosome segment in a different
chromosome.
Triallel Analysis. A genetic analysis based on the all possible
three-way crosses among n parents.
Triple Test Cross Analysis. A genetic analysis based on the pro-
genies derived by crossing a sample of randomly selected F
plants with the parental Fi and the two homozygous parents.
Triploid. An individual having three identical or distinct genomes.
Trisomic. An individual having one chromosome in addition to the
normal diploid chromosome complement of the species (2 n+l).
Trivalent. Structure formed by pairing among three homologous
chromosomes during meiosis.
Uniform Regional Trials, See Coordinated Varietal Trials.
Univalent. During meiosis ; an unpaired chromosome.
Variability. The amount of variation present among the members
of a population or species ; may have reference to one cr more
characters, and at genotypic or phenotypic levels.
Variance. Average of the squares of deviations of the observations
of a sample from the mean of the sample drawn from a popu-
lation. [2{x-xflN]
620
Plant Breedtn S • Principles and Methods
Variation. Differences among individuals belonging to a ,
species or to different species. Variation mav herfi » a Sln 8*e
environment, or. both genotype and S”nmSt 8 " , ° 1 >’P'.
Varietal Cross. See Population Cross.
Varietal Mixture. A population of a self-nniiin*^
tutedby mixing the seeds oftwLr ^ morf °° as&
some other proportion. varieties in equal or
Variety. In plant breeding, a strain released for commercial ra-
tion by a variety release committee* in botany a sJwkL* 5 U ^
species based on form, function or both. s ™"TOioa of
*’ rod "' ,iOT new individuals from vege,,.
^resistant and susceptible varietw^tbe^oltk 0 * 1 attacks both
, ' h,m tie 01t “ “®W»- (</. Resistance BrLS.gSS 8 ''"” 1 '
race) toettaut? ufSnSf the*!™” ^ pa,llot J'P® or physiological
Vertical Resistance. Resistance, usually immunity -w„ ^ . ,
race/pathotype j the specific virulent a Vlfu?en ^
susceptible response. Generally, coitrouS fc^|° 0 |g s f odaces
IF/* Cross. Cross between two soeciec n f ♦»,„
different genera Distant Cross). ? ** me geaas 0r of
World Collection. See Gene Bank.
X* See Basic Number .
Xs First, second third Mr
irradation (with X-rays 'or some ' othS mm’d° Sp ‘ am,y - a!K '
obtained through seifiig or cloMlmaWplSon'”" 0 Iadia,iOT >
seed tissues {embryl'S endosperm™ * r * , ° f lle ” ot yP' of
garnet"' im “ dia,e 1>r °' te ° f «»» »f male and female
Zygotene. A stage during Prophase I ofmeiosis durino t.
logons chromosomes pair with each other. ■“ ® ic ^ ^omo-
Index
Antibiosis, 380, 381, 400-402, 409, 410
Apogamy, 42
Apomixis, 40, 41, 42, 51, 366, 463, 489
Apospory, 42
Apple, 19, 22, 25, 36, 139, 356, 366, 375, 403,
405, 409, 412
Apricot, 20, 22
Arachis, 489, 492
Areeanut, 585
Argemone maxicam, 36
Aihar, 6, 17, 20, 25, 50, 61, 342, 515, 551,
555, 580
Arithmetic mean-see, mean
Arrow root, 23
Artichoke, 22
Jerusalem, 23
Arum, 356
Arwi, 41
Asparagus, 22, 49 ' .
Asters, 34'.
Asynaptic 445, 452
Auerback, 416
Autogamy-see, self-pollination
Avacado, 23
22, 480, 552
abbysinica, 19
barbate
byzantina, 141
nuda ~ see naked oats
saliva also see, oat, 488
sterilis, 488
Avirulent, 374, 375, 379-386
Ablemoschus esculintus, 499, see also bhindi.
manihot, 387, 494
Acada, 584
Acclimatisation, 29, 30
Adaptability, 120
Addition line, 253
alien, 470, 483, 484, 486
monosome, alien, 484, 486
Additive gene action, 91, 92
Ad hoc projects, 579, 588, 589
Adventive embryony , 4 1
Aegilop
caudata, 64, 68, 489
ovata, 64
speltoides, 238, 466
squarrosa, 466
umbell ulata, 477
ventricosa, 480
Agriculture Universities, 574-579, 506-509
Agrobacterium, 507
Agrodimatic zones, 511-515, 569, 579, 588
Agroecological zone, 574
Agropyron, 489
Agropyron repens, 500
Alchemiila, 42
Alfalfa, 19, 22, 49, 103, 149, 179, 181, 183,
250, 345, 387, 388, 403-405, 406, 408, 409,
410, 412, 457, 499
Alkylation, 423, 424
Allard, R.W. 122, 149, 194, 198, 286-287, 455
Allium, 20, 22, 42
Allogammy — see, cross pollination
Almond, 22
Alpha-pa rticles-see, alpha-rays
Alpha-rays, 418, 419
Alsike clover, 459
Alter mria, 516
AmorphophaUus, 583
Amphidiploid, 444, 463, 468-70, 475, 490
Anderson, 103
Annatto, 25
Anther cultur-see, culture, pollen
Anthesis, 46, 59
Antennaria, 42
Bab cock, 56
Backcross, 62, 63, 67, 68, 76, 140, 237-253,
255, 265, 266, 269, 274, 339, 340, 390,
407, 433, 434, 449, 450, 470, 486, 489
method, 237-253, 255, 389, 390, 484, 505
pedigree method, 250, 255
Bahl, 293 •
Bajra, 3, 5, 64, 65, 68, 90, 95, 136, 345, 560,
580, 586, 593
aneuploidy, 445, 446
breeding, 300, 310, 353, 369
622
disease, 13, 393
disease resistance, 252, 340, 344, 389
emasculation, 147, 148
evolution, 18
germplasm, 33
heterosis, 183, 191
hybrid, 11, 13, 35, 252, 328, 330, 331, 343,
389, 518, 540, 547, 557, 560, 561
inbreeding depression 179
origin, 23
pollination, 49, 148, 149
polyploidy, 455
seed production, 65, 69, 337, 539
trial, 511, 547, 560, 561
variability, 103
varieties, II, 35, 310, 344, 584, 594
weeds parasitic, 412
Bajra-Napier hybrid, 490
Balance, heterozygous, 180, 181
homozygous, 180, 181
Banana, 19, 22, 36, 49, 356, 357, 366, 367,
455, 457-459
Barber, C.A., 11,581
Barley, 77, 79, 80, 81, 128, 132, 133, 207,
222, 489, 499, 500, 503, 504, 592, 594
aneuploidy, 446, 452
breeding, 127, 202, 221, 229, 234, 262
281, 289, 292
disease, 378, 391
disease resistance, 81, 380, 387, 388
dormancy, 6, 17
emasculation, 146
evolution, 18
. germplasm, 31, 33
heterosis, 188, 191
insect pest, 397, 403, 404, 412
mutagenesis, 416, 434
origin, 22
pollination, 47, 148
pollyploidy, 453, 460
quality, 5
seed production, 61, 542, 552, 557
varieties, 207, 234, 412, 434, 484
Barnes, 527
Barrus, 370, 373
Beal, 182, 329
Beans (also see rajma) 128-130, 593
broad, 22, 23, 343, 382, 542
jumping, 29
lima, 232, 77, 383
Beets-see, sugarbdet
Belling, 444
Berseem, 29, 459
Plant Breeding : Principles and Methods
Beta-rays, 418, 419
Beta sp., 488
Beta vulgar is -see sugarbeets
Bhindi, 145, 203, 387, 388, 493, 494 534
Biffen, 370, 381
Biometrical genetics, 100
Biometry, 100
Biotype-see pathotype
Biparental cross analysis, 118-120
Black-gram -see urd
Black pepper, 22, 29
Blakeslu, 370, 444
BOAA, 7
Boerma,' 270, 278, 292
Borlaug, 9, 256
Botrytis cineria, 13
Bradshaw, 122
Brassiea , 20, 22, 45, 49, 57, 58, 59, 60, 103
401, 403, 466, 468, 488, 503, 568
campestris , 22, 207, 310; 434, 466 468
471, 472, 479, 488, 490
carinata , 468
chine ns is, 459
hirta, 434
japonica, 22
juncea, 25, 64, 207, 310, 468
napus, 64, 466, 468, 471, 472 476 488
490
nigra, 22, 468
oil, 7
oleracea , 55, 57, 339, 459, 466, 468 472
476, 479
rapa-also see, turnip., 476
Braun, 496
Bridging species, 469, 470, 480, 490
Brim, 259, 272
Brinjal, 20, 47, 145, 191, 265, 267
Broccoli, 18
Brown sarson
Bruce, 186
Brussel’s sprouts, 18, 191, 339, 400, 401
Buck wheat, 20; 53
Bulbil, 41
Bulk method, 224-235, 255, 260, 269 272
277, 286-295
-pedigree method-see, masspedigree
method
Bunda, 20, 41
ByJi, 277
: c ... : ; : || £
Cabbage, 18, 22, 25, 184, 339, 383, 387, 388,
Index
401, 455, 459, 461, 471, 475
Cacao, 345, 585
Cajanus eajan-see, arhar
Callus culture-see under culture
Camararious, 139
Camellia, 20, 25
Cannabis indica-see, hemp
Capsicum, 69
Cardamom, 41, 585
Carnation, 139, 475
Carrot, 22, 35, 179, 504, 505, 532, 534
Carihamus tinctorius-se& t safflower
Casali, 277
Cashewnut, 25, 267, 585
Cassava, 23, 49, 356, 583, 584, 593
Castor, 23, 45, 49, 61, 144, 310, 568
Cattle, 191
Cauliflower, 18, 25, 35, 534
Central Institutes, 26, 29, 33, 579-589
Centres, of
diversity, 23, 24
origin, 19-24, 25, 32-34
primary, 20, 22, 23
secondary, 20, 22, 23
Cercospora , 388
CGIAR, 591, 593, 594
Chasmogamy, 47
Cherry, 19
Chestnut, 22, 49 '
Chickpea-see, gram
Chillies, 23, 25, 34, 265, 534
Chimera, 358, 426, 429
perielinal, 426
sectorial, 426
Chinese tea, 20
Choo 288, 289
Chromosome, 3, 73, 80
aberration, 132, 133, 425, 433, 434, 505
banding pattern, 466
behaviour, 446, 455, 464
complement-see genome . •
doubling, 19, 59, 261, 262, 264, 282, 333,
444, 452-454, 457, 461-463, 468, 470, 479,
480, 481, 483, 484, 490, 491, 494, 496
elimination, 458, 477, 478
homoeologous, 447
homoeologous pairing, 447, 463, 464 486
487 ’ ’
homologous, 73, 80, 446, 463
homology, 442, 467, 482, 491, 493
pairing, 73, 238, 442, 455, 461, 464, 466,
48 1-483, 491, 492
iso, 445, 450
manipulation technique, 487, 491
reduction, 454
substitution, 447-450, 452
telocentric, 450
transfer, 448, 450, 470, 486, 491
translocated, 445
transmission, 446
Chromosome number, 460
basic, 442-444
gametic, 442
somatic, 442, 444, 470, 484 490
CXAT, 591-593
Cicer arietinum-also see, gram, 488, 489, 491
reticulatum, 488, 491
CIMMYT, 9, 24, 35, 203, 218, 300, 345, 388
471, 591-593, 595, 596
OP, 592, 593
Citrus, 20, 22, 26, 28, 32, 36, 41, 356, 366,
584
CitruIIus vulgaris- see, water melon
Clausen 444, 450
Oistogamy, 47, 50, 488
Clonal crops, 356-367, 415, 433, 434, 502
degeneration, 358-360, 365
propagation, 356-367, 460, 481, 502-505
selection-see, under selection
Clone, 135, 357-367, 433, 502
Clovers, 22, 55, 80, 147, 345, 346, 408
Cochliobolus-see, Helmintkosporium
Cockerham, 115, 272
Cocking, 479
Coconut, 22, 43, 49, 344, 345, 567, 585
Cocos nucifera-sce coconut
Coefficient of variation, 101
Coffee, 23, 26, 31, 33, 34, 36, 55, 457, 504
Colchicine, 452-454, 461, 462, 483, 484
Colchicum autumnale , 455
Cold tolerance, 217, 225
Coleus , 583
Collins, 187
Colocasia, 43, 356, 366, 499, 583, 584
Coloeasia amiquorum-see, bunda
Combination breeding, 139, 140, 144, 213
Combining ability, 113, 144, 313, 329, 333
334
analysis, 113
general 113, 114, 116,313,314, 317,319,
320, 321, 324, 329, 333, 334, 341, 346
350-353, 363, 366
specific, 1J3, 114, 116,310,317,319, 321
329, 335, 341, 348, 353, 363, 366, 386
test, 335, 346, 352, 353
Commodity committees, 567-570
624
Plant Breeding : Principles and Methods
Compton scattering, 419
Comstock, 119, 122, 316
Convergent improvement, 340
Cooper, 127, 270, 278, 292, 569-571
Coordinated projects, 5, 9, 12, 33, 300, 330,
510, 511, 516, 517, 519, 520, 579, 581-
588
Corchorus-szt, Jute
Correlation, coefficient analysis, 105-109, 130,
131,219,270,278, 306
Cotton, 6, 17, 19, 25, 29, 31, 50, 103, 141,
145, 183, 191, 207, 222, 282, 310, 328,
337, 341, 344, 381, 387, 388, 401-406, 409-
412, 436, 438, 466, 479, 483, 484, 489,
490, 493, 519, 520, 542, 555, 565, 567,
568, 581, 582
American, 11
emasculation, 267
heterosis, 11, 265
hybrid, 11, 61, 267, 339, 343, 472, 489,
520
insect pest 397, 398
origin, 22, 23
Covariance, 101, 107, 108, 111, 112, 270
Cowpea, 20, 35, 47, 203, 278, 593
Cramer, 398
Crepis , 42
foetida, 56
tectorum, 477
Cross, 138-153
complex, 32, 140-142, 229, 280, 288, 365
composite, 229, 234, 288
convergent-see, complex cross
distant, 459, 462, 490,. 500, 501, 507
double. 58, 65, 68, 115, 319, 328, 329, 330,
334-337, 339, 348 .
interspecific-see interspecific hybridization
poly-159, 329, 346, 352
population-see, varietal cross
reciprocal, 458, 482
simple, 140, 144
single, 65, 68, 319, 328, 329, 330, 334-
337, 339, 348, 351, 353
test, 329
three-way, 58, 65, 114, 329
top,, 329, 335, 346, 353
triple-see, three-way
triple-test, 119, 120 ' . :v
varietal, 319, 320, 329
Crossing over, 80, 81, 175, 242, 373, 455, 456,
483, 486, 492
somatic, 373, 505
CRRI, 31, 580, 582, 583
Cryobiology, 507
Cryptic structural changes, 481
CSC (Central Seed Committee), 557
CSC B, 557
Cucumber, 20
Cueumis , 20
Cucurbita , 20
Cucurbits, 49, 179, 180, 191
Culex pipiens , 482
Culture
anther, 261, 262, 264, 282, 283, 285, 286
496, 500, 508 * ?
callus, 453, 498
c ell, 496-508
embryo, 262, 479, 480, 496, 500, 501 507
508 * '
medium, 41, 496-499
meristem, 500-503, 508
organ, 497
pollen-see under anther
suspension, 453, 498
tissue, 41, 286, 496-508
Cummings, R.W., 575
Cumulative gene action -see, additive gene
action
Curcuma domesiica-s&e , turmeric
Cumow, 114
Currant, 139
Cytoplasmic
effect, 482, 493
incompatibility, 477-479
inheritance, 381, 382, 387
-nuclear interaction, 463
male sterility-see under sterility, male
transfer, 483, 489, 493, 507
D
D 2 statistic, 102, 103
Bactylis, 480
Dahiya, 280
Darwin, C, 178, 182
Date-palm, 41, 43, 49, 138
Datura, 42, 444, 445, 453, 477, 496
Dane us carota - see carrot
Daven port, 186
Davis, 115
De Jesus, Jr., 287
Deamination, 423
Deletion, 415, 423, 481, 482, 505
Dcpurination, 424
Desynaptic 445, 452
Dctasscling, 336, 337, 339
1
De Vries, H., 83, 415
Dewey, 107
Diallel 102, 110-113, 115, 119, 335
no,,m.ii5..„
, jekclive matkg,,sdbraie, 259,. 260
Dicaryon, 372,
Dicliny, 49
Dimer formation, 423
Dioecy (Dioecious), 17, 49, 50
Dioecious (Dioecy), 43
Dioscorea , 22, 356, 504, 583
Diploidized, 464
Diplospory, 42
Discriminant function, 105
Disease, 33, 256, 369-394, 404, 540
air-bom, 391
development, 375, 376
epidemic, 13, 217, 244, 245, 248, 377, 386,
391-394, 518, 534, 597
epidemic, artificial, 389-391, 510
escape, 376-378, 401
free stocks, 502-504, 508
hot spots, 518
immunity, 378, 379, 382
insect transmitted, 391
seed-bom, 391
soil-bom, 377, 390
susceptibility, 12, 13, 370, 377, 378, 392
tolerance, 378
Disease resistance, 6, 1 1, 78, 79, 81, 144, 370,
377-394, 409
breeding, 4, 13, 140, 237, 243-246, 248,
256, 388-390, 487, 506
evaluation of, 508, 510, 516, 522
genetics of, 381-387
horizontal, 378-380, 386, 387, 389, 392-
394
mechanism of, 380, 381
race-nonspecific-see, horizontal resistance
race-specific-see, vertical resistance
selection for, 216, 217, 225
sources of, 387, 388
test for, 390, 391
varieties, 6
vertical, 378-386, 392, 393
Disomic, 445, 448, 450, 451
DNA, 415, 421, 423, 424, 507, 508
Dockage, 551
Domestication, 2, 8, 15-17, 126
selection during, 2, 126
Dominance, 74, 161, 167, 272
complete. 111, 112, 152, 319, 325
degree of. 111
gene action, 91
hypothesis, 186-189
incomplete-see, partial dominance of linked
genes hypothesis, 187
over. 111, 112, 187-189, 319, 325
partial, 111,112, 319, 325
Dominquez-Jimenez, 275
Donar parent, 63, 237-253, 450
Dormancy, 6, 492, 500, 501
Driscoll, 492
Drosophila , 172, 173, 175, 325, 416, 421,
507
Duplex, 455, 456
Duplication, 415, 481, 482, 505
EC, 129
Early testing, 334
East, 53, 84, 86, 133, 178, 183, 187, 188, 310
Eberhart, 121
Eichornia crassipes , 36
Elastic scattering, 419
Elliot, 149
Emasculation, 144-148, 267, 342
genetic, 147, 336
hand, 11, 145, 259, 336, 339
Embryo culture-see, under culture
Empig, 292
Endosperm absortion, 477, 480, 496, 500, 507
Entry 515-524
EpUobium , 482
Epiphytotics-see, epidemic, disease
Epistasis, 91, 188, 189, 272, 274, 281, 319
Erikson,. 370
Erucic acid, 7
Eucalyptus, 36, 584
Euphorbia lathyrus-scc , milkweed
Evans, 287, 289
Evolution,: '- 1 -19
Excitation, 418, 419
Explant, 497, 500, 502
Exploration, 27, 32, 33, 35
Expressivity, 77, 78
Fagopyrum esculentum-ste, buck-wheat
Fairchild, T., 3, 139, 475
FAD, 27, 28, 583, 591
Fasoulas, 218
Fehr, 291, 292
t
ni
3ft
m
626
Plant Breeding : Principles and Methods
Feadt, 283
Fernandez-Maitinez, 275
Fertilization, 40, 41, 44, 52, 53, 73 74
477
failure of
Fescue, 103
Festuca , 480
Festuca-Lolium hybrids, 471, 491, 494
Field inspection, 536, 537, 540, 545-550
558-562
Fig, 22, 366
Finger millet, 25
Finley, 120
Fisher, 91, 101
Fitness, 160, 162, 286-288
Flor, 370, 382
Ford foundation, 11, 592, 593
Foroughi-Wehr, 283
Foxtail millet, 47
Fragaria- see, strawberry
Free man, 122
Free radicals, 421-423
Frey\ KJ., 291, 488
Fritilkri a, 463
Fusarium
476,
Gajendra Gadkar, 574
Gametogenesis, 44, 74
mega, 44, 45
micr, 43, 44
Gaertner, 139 \
Gallon, 83
Gamma-rays, 419, 432, 438, 453, 487
Gamma-garden, 431, 432
Garber, 310, 345
Gardner, 218, 305
Garlic, 22, 41
Gautheret, 496
Geitonogamy, 46
Gene, 3, 73*74
action, 91, 272, 316
bank, 30, 32
frequency, 155-171, 275, 286. 290 311
351
function, 72, 73
ghost-, hypothesis, 380
interaction,' 79, 80, 381, 382 \
lethal, 150, 178. 180, 186, 417, 477-47#.
modifying, 57, 69, 78, 79, 245, 382, 403
mutator, 416
oIigo-73, 76, 77, 78, 79, 133, 379, 380,
381, 386, 387, 403, 404, 409, 410 424
plasma, 68, 387, 404, 415
poly, 57, 73, 87, 88, 379, 380, 381 m
387, 404. 405, 409, 410, 424 ’
pool, 155
restorer, 50, 62, 65, 66, 67, 69, 265
sanctuary, 27, 32
sublethai, 417
556, subvital, 417
transfer, 2, 18, 19, 65, 67, 238, 251 son
507* 447 ’ 47 °* 4?6 * 48 °’ 483 ’ 48 6-493, 505*
Gene-for-gene relationship, 370, 379, 380
molecular basis for, 384-386
Generation mean analysis, 102, 117
Genetic
background, 73, 380, 416, 433, 44g
base, 12, 13, 392, 469, 471
buffering, 492
diversity, 102-104
drift-see, random drift
erosion, 12, 30, 35
homeostasis, 120
imbalance, 477, 478
load, 180
vulnerability, 12, 13
Genome, 442-446, 463, 466, 468, 477, 492
Genotype, 72
frequency, 155-159, 162, 275, 286 28
290, 301, 311
Genotypic dishormony, 477, 478
Germplasm, 26-36, 260, 330, 433 502 50'
570, 585 ’
collection, 8, 12, 30-33, 387, 405 433 57f
588,594
complexes, 345
conservation, 25, 30, 502, 503, 507 508
594, 596
conversion, 239
exchange, 502, 503, 508, 585, 596
repository, National, 31
storage, 31, 32
Gerstel, 56 .
Ginger, 41, 356, 502, 585
, Glycine, 492, 552
Glycine max-also see, soybean, 492 j
Good speed, 444
Gopher plant, 16
Goss, I, 139
\ Gossypium t 582
anomalum , 64, 406
; arboreum, , 22, 64, 141, 251, 310, 401, 468,
• x 488
Index
627
507, 508
doubled, 262, 263, 269, 281-286, 457
Hardy, 155 _
Hardy-Weinberg
law, 155, 156
equilibrium, 156, 159, 161, 162
Harlan, 225, 234, 476, 487
Harrison, 172
Harvey, B.L., 288, 289
Harvey, 316
Hayes, 310, 345
Hayfield tarweed, 179
Hayman, 91, 101, 110, 111, 116
Homozygosity(-ous), 40, 41, 47, 51, 75, 86,
130-132, 151, 160, 164, 165, 177, 181, 186,
240, 255, 257, 263
Honeycomb design, 218, 219
Hopkins, 306
Hordeum
hulhosum, 262, 282, 283, 285, 458, 478,
501, 508
jubatum
nodosum
vulgare, 45, see, barley, 458, 418, 480,
500, 501, 508
Host, 368-393, 397-412
differential, 373, 374
armouriamm , 406
Haynaldia villosa, 470
- If
barbadense , 11, 19, 23, 267, 468, 489
Hazal, 109
davktsoni , 477
Heiraceum, 42
,1
gossypioides, All
Hetkmthus-SQG , sunflower
# i
herbceum, 22, 25, 252, 438, 468
Helminthosporium , 1 3, 64, 69, 305, 370, 384,
k : -j
hirsutum , 11, 19, 23, 64, 141, 208, 209,
385, 387, 388, 506
222, 238, 251, 252, 267, 310, 388, 389,
Helvo, E., 527
■ : W[ ?
405, 468, 470, 471, 486, 488, 489
Hemp, 22, 43, 49, 180
t : '
purpureascens , 23
Heptaploid, 444
y ■
raimondie, 468
Hermaphrodite, 42, 46, 49
].■■■'■
thurberi, 468, 470, 488
Heritability, 95-97, 105, 109, 118, 134-136,
■ III
tomentosum, 406, 468
160, 162, 164, 167, 169, 172,216-219,238,
fv.
Goulden, 272
245, 248, 261, 270, 277, 291, 301, 302,
If''
Gram, 13, 20, 22, 23, 47, 203, 207, 221, 293,
305, 306, 312
r
1
412, 499, 511, 536, 551, 593, 594
broad sense, 96, 118, 136
Grapes, 19, 22, 24, 34, 41, 49, 139, 398, 406,
narrow sense, 96, 118, 136
411, 412, 460, 487, 584
Herrera, 287
IS-;"
Green gram-see, mung
Heterobeltiosis, 181
Green revolution, 596, 597
Heterocaryosis, 371-373
1
Griffing, 110, 113
Heteroploidy, 442-444
1
Ground nut, 23, 25, 31, 34, 47, 207, 377, 388,
Heterosis, 48, 50, 103, 177, 178, 180-181, 191,
457, 492, 537, 542, 560, 593, 594
263, 265, 273, 316, 319, 325, 328, 343,
f
Gaur, 47
346
iV
Guava, 23, 25, 34
average, 181
fly,
s ■
Guha, 496
breeding, 310^311
Gulmohar, 34
economic, 1 8 1
pi
: 1 ' ;
I
Gustafsson, 188, 416
generic basis of, 184, 186-189
useful, 181
H
Heterothallic, 372
Heterozygosity(-ous), 40, 41, 42, 50, 51, 75,
■
’$&■ . . .
Haberlandt, 496
86, 130-132, 144, 151, 160-166, 177, 180,
■ h ■
HabibuHa, K.B. Sir M., 567
181, 186, 240, 250, 257, 260, 263, 348-
f iS
Haddad, 288, 293
350, 352, 356
:
Hallauer, 305, 320, 321
Hevea sp., 16, 33, 34
v ilpliifej
Hallet; 127, 202
Hexaploid, 441, 471
Hamblin, 287, 289, 290, 294
alio, 444, 470
I
Hannah, H.W., 575
auto, 444
1
Hanson, 110
Homocaryosis, 372
i
Haploid, 261, 262, 263, 285, 333, 442, 452,
ACA A£ o Ar\s a .'A**'
Homothallic, 372
fev a
0
'll. 28
>n
Plant Breeding : Principles and Methods
-tester-see, differential host
Hull, 310, 316
Hume, A,0„ 565
Hybrid,
aritificial, 3, 40, 183, 284
distant, 461, 500, 507
interspecific, 182, 267, 433, 461, 463 467-
469, 475-494, 500
intervarietal
necresis, 150
product, 190
seed, 145, 191, 335-339
seed production-see, under seed production
sterility, 480-482, 491
variety-see, under variety
vigour-see, heterosis
Hybridization, 8 , 24, 33, 103, 138-153 174
175, 177, 202, 21 3, 255, 262-265, 367, 388-
390, 406, 407, 424, 433, 438, 460 464
471
distant, 8 , 140, 141, 150, 261, 262 475 -
494, 581
intraspecific-see, intervarietal
intergeneric, 141, 475
interspecific, 18, 19, 68 , 133, 134 141 165
182, 282, 283, 286, 365-367, 387’ 444 ’ 45 s’
468,475 ’ ’ ’
intervarietal, 8 , 140, ISO, 165 182 183
433, 475, 476, 482 ’ ' ’
introgressive, 18, 19
natural 18, 332-134, 202, 530
somatic, 491, 506, 507
Hydari, S.A., 567
Hypersensitivity, 380, 381
isolation, 34, 316, 319, 331-334
line, 134, 178, 179, 183, 184 U
273, 554
maintenance, 58
second (higher) cycle, 331, 339
short-tenm, 346, 351
Inbreeding, 3, 42, 50, 51, 58, 130, 16
162, 164-66, 177-181, 186, 189 19
300, 301, 303, 309, 311 313 ’ 3J .
328, 331. 348, 351, 352, 358, 362,
coefficient, 292
depression, 42, 47, 50, 51, 52, 162
181, 182, 184, 189, 265, 300,’ 301
328. 356
Incompatibility, self, 49, 50, 52 60 79
336-339, 363, 372, 38?*
elimination of, 17, 59
gametophytic, 49, 53, 55-60
heteromorphic, 53, 54
homomorphic, 53, 57
mechanism, 57
mechanism, 57
overcoming, 59, 60, 458
sporophytic, 49, 53-59
Indigo, 22
indigofera , sp., see, indigo
Infector, 248, 391, 392
Insect pest, 4, 33, 397-412
avoidance, 401
genetic variability, 398-400
outbreaks, 410, 41 1
losses due to, 398
pest 397-412
pesticide-resistant-397
tolerance, 400, 401, 410
vector, 377, 391, 398
Insect resistance; 6 , 397 , 399
durability of, 409, 410
breeding for, 4, 13, 369, 406, 407
evaluation of, 518
genetics of, 403-405
mechanism of, 405, 406
sources for, 405, 406
tests for 407, 408, 410
varieties, 6 , 397, 399, 411, 412
Insertion, 423
Interallelic interaction-see, epistasis
Introduction, plant, 8 , 24-36, 208 222 38
406, 585, 596
primary, 24, 34
secondary, 24, 34
restriction on, 29
Imrogression, 488, 489
JAKI. 25, 26, 432, 566, 568-570, 580
IBPGR, 27, 30, 592, 594
IC, 29
ICAR 5, 9, 25, 26, 33, 527, 528, 536
556,566-589
ICARDA, 592, 594
ICRISAT, 300, 345, 353, 592, 593 59‘
ETA, 591, 593 ' '
DLRAD. 592, 594
ILCA, 592, 594
hbred, 3, 65, 103, 274, 282, 316 328
331-353,359 '
development, 164, 331-334 4%
evaluation, 334-336
first cycle, 331
improvement of, 330, 331, 339, 340
Index
Kearsey, 119, 120
Keeble, 186
Kempthome, 113, 116
Ketata, 120
Khalifa, 287, 290
Khesari, 7, 23, 47
Kihara, 466
Knight, A., 3, 127, 139, 370
Knott, D.R., 270, 277, 279, 281
Koelreuter, I, 3, 139, 182
Kothari, D.S., 575
Kumar, 270, 277, 279, 281
Inversion, 415, 481
In vitro techniques, 416, 462, 496-508
Ionisation, 418, 419, 421-423
Iris, 19, 500, 501, 508,
Irradiation, 424, 425, 431, 432, 453
recurrent, 431
IRRI, 10, 31, 591, 592, 596
Isogenic lines, 65, 76, 77, 239
Isolation, 50, 353, 431, 538, 554, 557-562
distance 536, 538, 539, 551-552
IW, 29
Iain, 288, 289
Jenkins, 313, 334
Jennings, 287
Jensel 259, 260
Jenson, 256
Jha, A.N., 575
Jinks, 111, 116, 117, 119, 120, 121, 273, 274,
278, 280
Johannsen, 3, 127-130
Johnson,' 1 10, 528
Johnson grass-see Sorghum halepense
Jojoba, 116, 117 , 187, 310, 330
Jowar, 3, 5, 64, 65, 207, 568, 569, 580, 581,
586, 592, 593
emasculation, 146, 148, 337
evolution, 18
disease resistance 383, 388, 391
germplasm, 31, 239
haploidy, 454
heterosis, 183, 184, 191, 265
hybrid, 11, 35, 185, 265, 266, 328, 330,
331, 343,555, 557, 559, 560
inbreeding, depression, 118, 179
insect resistance, 401
introduction, 25
.origin, 23
pollination, 50, 149
: polyploidy, 460
seed production, 65, 69, 534, 539, 540, 542,
555, 559, 560
trial 511, 516
variability, 103, 594
varieties, 11, 203, 344
weeds, parasitic, 412
Jute, 17, 29, 145, 207, 436, 438, 471, 491,
519,55,56 7
Lac, 567
Lactuca sativa- see, lettuce
Laiback, 496
Lantana camara , 36
Lathy r us, 22
Ldlhyrus sativus- see, Khesari
Lavender, 22
LD, 50, 425
Le Couteur, 3, 127, 202
Lentil, 22, 23, 47, 287, 293, 594
Lethal gene-see, under gene
Lettuce 22, 35, 405, 412, 489
Lewis, 52, 56''
Lilies, 19 ■' ;o " =■, ;; r; : 37
Lilium , 57
Lima bean-see, under bean
Line X tester analysis, 102, 110, 115, 116
Linkage, 77, 80-82, 106, 113, 131, 132, 151,
1597 229, 249, 274, 281, 325, 407, 417,
433, 456, 482, 483, 492
coupling phase, 81, 82, 274, 282
disequilibrium, 319
group, 447
repulsion phase, 82, 82, 175, 188, 189, 255,
274,282
tight, 77, 81, 238, 239, 242, 245
linseed, 18, 22, 23, 29, 33, 103, 203, 273,
382, 388, 557, 580
Linum-alsQ see, linseed, 496
Lit chi, 25, 356
Lodging, resistance, 217, 225
Lolium, 480, 552
Lonnquist, 219, 308, 320
Loquat, 25, 356
Lord Curzon, 566
Lord Linlithgow, 566
Lord Mayo, 565
La. 107
0
Plant Breeding : Principles and Methods
Lupins,. 22.
Lutz, 444
Luxuriance, 182
Lycopersicon, 53, 57, 60
esadentum , 250, 476, 480
hirsutum , 488
peruvianum, 250 .
pimpineilifolium, 476, 481
Lytkrwn, 53
Mac, 288, 289
Macromutation-see, under mutation
Me Ginnis, 270
Maheshwari, 496
Mahogany, 25
Maize, 3, 5, 6, 40, 45, 62, 68, 79, 144, 147,
149, 171, 172, 333, 340, 352, 353, 477,
479, 481, 488, 499, 580, 581, 5S6, 592,
593
aneuploidy, 445, 446, 452
breeding, 109, 218,* 219, 221, 300, 305,
306, 308-3 10, 3 19, 320, 324, 325, 345, 353
composites, 330, 331
disease, 13; 370, 392, 393,. 506
disease resistance, 385, 387, 392, 506
emasculation, 148, 336
evolution, 18, 19
fertility, male, 64, 65
germplasm, 26* 31, 592
haploid, 457
heterosis, 182, 183-184, 188, 189, 191,569
hybrid, 7, 11, 13, 35, 328-343, 539, 555,
557, 559, 570
inbreeding depression, 178, 179, 181, 182
insea resistance, 403, 405, 410
introduction, 25, 33, 34
mutation, 133, 416, 421
origin, 20, 23
pollination, 46, 49, 69, 148, 301
polyploidy, 453 ,*455, 459
reproduction, 42, 43, 488
seed production, 65, 337, 532, 534, 537,
539, 540, 542, 550, 552, 555, 558. 559
variability, 103
varieties, 9, 10, 11, 169, 310, 343,, 350,
351,353
Major gene-see, oligogene
Maintainer line, 61, 62, 63, 266
Malaria 188, 190
Male sterile
cytoplasm 13, 62-69, 266, 387, 393, 506
line, 60-69, 265-267, 288, 337, 338, 344
389, 393, 489
Mai us, 42, 365
Mangelsdorf, 53
Mangifera , 32
Mango, 28, 4!, 49, 356, 366, 584
Martin, 275, 277 1
Mass method-see, bulk method
Mass-pedigree method-see, under pedigree
Mather, 91, 101, 117, 118, 172, 180
Mating,
random, 157, 158, 159, 163, 164, 166, 167,
313, 348-350 -
system 163-166
Matzinger, 259, 272
Maxwell, 397
Mean, 101, 134, 135, 272, 281
Mechanical mixture, 132-134, 144,. 358, 431,
530, 540, 550, 554, 555, 558-562
Medicago
disciformis, 402
sativa-see, alfalfa
Megagametogenesis-see, under gametogenesis
Megasporogcnesis-see, under sporogenesis
Melilotus, 479
Melons, 23
Mendel, G. r 3, 73, 83; 139" ^ '
lays of inheritance, 3, 73, 83
Mendelian population-see, random mating
peculation
Mentha, 356
Mericloning, 502
Merisiem, cultur-see, under culture
Mesta, 582
Metasenia, 138
Metroglyph analysis, 102-104
Microcentres, 24
Micromutation-see, under mutation
Microgametogenesis-see, under gametogenesis
Microsporogenesis-see, under sporogenesis
Migration, 155, 159, 165
Milkweed, 16
Minor genes -see, polygenes
Mint, 41
Miranda, 305, 320, 321
Modifying, genes -see, under gene
Moll 109, 122, 321
Monoecious (monoecy), 42, 49, 50
Monopioid 442, 452, 455, 457, 460
Monosomic, 443-446, 448-450, 452
analysis, 445, 450
double, 443
Moth, 47
■ ■" ' * ‘ '
Index
63 !
Muller, 416
Muehl bauer, 287, 292, 293
Multiple factors-see, polygenes
Multiple factor hypothesis, 84, 87
Mumaw, 287
Mung, 6, 17, 20, 22, 25, 47, 87, 103, 104, 203,.
207, 275, 288, 294, 377, 552
Muse, 32
Musk melon, 22
Mustard, 25, 434
Mutagen, 416-439
chemical 418, 423-426, 431, 438, 439
ionizing, 418, 419
nonionizing, 418-421
physical, 417-423 , 425, 437-439
Mutagenesis, 425-432, 454
directed, 159, 432
Mutant, 150, 424, 457
cytoplasm, 64-69
isolation (in vitro), 505, 506
varieties, 434-439
Mutation, 8, 18, 30, 35, 132.134, 155, 159,
371, 389, 415-439, 454
breeding, 416, 424-439, 505
bud -see, somati c mutation
chromosomal, 412, 423, 424, 505
cytoplasmic, 68, 415
frame-shift, 423
frequency, 416, 417, 424, 432, 434
gene, 415, 424, 468, 506
induced, 68, 150, 202, 387, 389, 416,
417
' induction-see, mutagenesis
: macro, 18
micro, 18
plasma gene, 505
somatic, 358, 415
spontaneous, 18, 68, 133, 134.. 150, 202,
388, 389, 416, 417, 489
Mutator gene-see, under gcne :
Naudin, 139
NBPGR, 25-28, 31, 32, 585
' Nebel, 444
N euros par a , 188, 507
Neutron, 419
fast, 419
thermal, 419
Nicoiiana , 42, 49, 53, 453, 459, 478
bigelovii, 64, 489
glauca , 506
glutinosa , 444, 484, 490
langidorfii , 506
longiflora , 84, 85, 238
me gale siphon, 64, 489
otophora , 480
plumbaginifolia, 489
rependa, 480
rusiica, 19, 133, 209, 274, 278, 280
sander ae, 53
sylvestris , 19, 467, 468, 470
tabacum, 19, 64, 238, 282, 444, 464, 467,
468, 470, 480, 484, 489, 498, 506
tomentosa , 19, 467, 468
Nilsson-Hhlc, 3, 83, 84, 224, 416
Nitch, C., 496
Nitch, J.P., 496
Noblisation, 10, 11,250
Nonaccepiance, 400-403, 410
Nonpreference-see, nonacceptancc
Nonrecurrent parent-sec, donar parent
North Caroling design, 118-120
NSC, 216, 520, 526, 528, 530, 531, 537, 545,
555, 556
Ntare, 278
Nufliplcx, 455, .456
Nullisomic, 443-450
analysis, 445, 447-450
Gat, 19, 22, 31, 33, 35, 47, 84, 127, 132, 141,
145, 148, 291, 383, 385, 388, 391. 393,
408, 412, 445, 446, 464, 483, 484, 488,
499
naked, 20
Octaploid, 444
alb, 444
auto, 444
Oenothera, 57, 59, 60, 444,-482
Off-season nursery, 220, 221, 231, 232, 251,
263, 286, 294-296
Oil palm, 35, 488, 489, 585
Oka, 481
Okra, 23, 33 * ' ■
Oligogcnc-scc, under gene
Onion, 22, 23, 35, 41, 179, 180, 184, 191, 337,
343, 487, 534
Opium poppy, 20
Orange, 20, 22
Orchid, 500, 501, 508
Organogcnesis-sec, regenerat ion
Orolxinche , 412
632
Oryza, 32
perennis , 475, 490
.M/iva-also see, rice, 475, 490
var. indica, 481, 490
var. japonica, 481, 490
Out-crossing-see, natural hybridization
Over dominance hypothesis, 187-189
P
Painter, 400
Pair production, 419
Pal, B.P., 570
Palmer, 259
P attic wn miliacewn , 20
Panmietic population -see, random mating
population
Pant, Pandit G.B., 575
Papaver , 53
Papaver sominiferum-sze , opium poppy
Papaya, 23, 25, 34, 43, 49
Parasitic weeds, 412
Parent-off spring regression, 96
Park, 282, 283
Parker, M.W., 570
Parlevlict, 380
;; Parthenccaipy, 366
Parthenogenesis, 42, 133, 333
Partkenium, 56, 463
Path coefficient analysis, 17, 105, 107-109
Pathogen, 256, 257, 369-394, 398, 399, 534
Pathogenedty, 370, 372, 375
Pathotype, 370, 374, 375, 379-386, 389-412
Pawar, 293
Peach, 20, 139, 356
Peanut-see, groundnut
Pear, 19, 20, 22, 25, 139, 356, 366
Pearl millet-see, bajara
Peas, 18, 22, 23, 34, 35, 47, 74, 75, 445 446
488, 499, 515, 552 * ’
vegetable, 34, 516
Pedigree, 210, 21 1, 520
mass, 225, 234, 255
method, 140, 210-222, 224, 231, 233-235,
240, 250, 252-253, 255, 259-261, 269-272*
277-281,283,287,290-296,339,389 407*
490
record, 210-213, 221, 233, 272, 281
Pellew, 186
Penetrance, 77, 78
Penicillium, 2
Pennisetum americanum-szz. also bajra, 488
Pentaploid, auto-444, 470
Plant Breeding : Principles and Methodi
Peppers, 489
Peppermint, 22, 487
Periconia cercinati , , 203, 388
Perkins, 121, 122
Persian clover, 22
Peterson, 291
Petunia , 53, 57, 59, 459, 506
P ha laris, 53
Phaseolus sp., 20, 294
Phenotype , 72, 73, 90-91
Phleum , 480
Phlox, 34
Photoelectric effect, 419
Photoinsensitive, 6, 10, 239
Phylaris minor, 36
Phylloxera vertifoliae, 398, 406, 411, 412
Physalis, 53
Physiological race , 370, 373-375, 378-386
Phytoalexin, 381
Pigeon pea-see, arhar
Pineapple, 23, 25, 34, 55, 366
Piper nigrum, 22
PIRE.COM, 568, 569, 581
Pistachionut, 22, 43
Pistillate, 42, 49
Pleiotropy, 61, 76, 77, 106, 417
Plum, 19,20
Pod com, 18
Pollen culture-see, under culture
JPpHinatiort, 46-52, 148, 267
artificial 3, 138
bud, 59
-control, 51, 52, 347, 352, 353
cross, 29, 46, 47, 48, 49, 50, 51, 52, 133,
138, 144, 146, 147, 149, 198, 265, 356,
460
double, 60
hand, 11, 265, 267, 458, 554
mode of, 29, 46, 50, 51, 52
natural, 61, 69
often cross, 48, 50
open, 345, 351
self, 29, 30, 46, 47, 49, 50, 55, 130-132,
138, 144-147, 149, 151, 198, 356, 460
Poly cross-see, under cross
Polygenes-see, under gene
Polygenic inheritance, 84, 87
Polyploidy, 17, 19, 357, 366, 416, 442-472,
477, 492
alio, 8, 19, 444, 461-472, 483, 490, 491
aneu, 442-452, 455 -
auto, S, 19, 444, 452, 454-461, 479,
eu, 443, 444
SMS
si mi
Index
633
Pomegranate, 22
Pooni, 273, 274, 280
Popcorn, 539 v
Population
breeding, 255, 257-261
genetics, 155
improvement, 263, 300-325, 330, 348, 352,
353
Potato, 17, 29, 34, 59, 145, 184, 357, 366,
479, 480, 484, 489, 499, 502.. 503, 505,
583 , 586, 593
breeding, 46, 358, 365, 367
disease, 36, 373, 377, 379, 380, 391, 392
disease resistance, 373, 380, 382, 383, 386,
388, 392, 487
evolution, 19
germplasm, 26, 31, 583, 593
insect resistance, 400, 401, 406
introduction, 25, 33
origin, 23
polyploidy, 457, 459
propagation, 41, 356
seed, 377, 526
seed production, 557, 561, 562
varieties, 358, 365, 367, 386, 392, 583
Poultry, 191
Powers, 181
Prepotency, 164, 165, 363
Prevost, B. 370
Primula, 53
floribunda, 461
kewensis, 461
verticillata, 461
Progeny test, 3, 76, 127, 194, 198, 202, 208,
300-301, 306, 308
Propagation, vegetative, 19, 128, 500-502, 504,
505, 508
Propagule, 27, 28, 29, 425, 502
Prosb millet-see, Panicum miliaceum
Protandry, 49, 50
Proiogyny, 49, 50, 147
Protoplast, 497, 506, 507
Pr units, 20, 22, 501
Pseudofertility, 59, 60
Pseudomonas t abaci , 506
Pumpkin, 23
Pureline, 92, 127-130, 132-136, 194, 195, 198,
201-209, 256, 357, 359, 496
origin of variation in, 132-134
. selection- see, under, selection
theory, 3, 128
variciics-sec, under varieties
Pyrus, 20, 22
Quadriailel analysis, 115
Quadruplex, 455, 456
Quality test, 5, 217, 510, 518, 519, 522
Qualset, 272, 287, 290, 293-295
Quarantine, 27, 28, 36, 370, 503
Quinine tree, 23, 25
Radhakrishnan, 5, 575
Radish, 20, 22, 35, 49, 57, 461, 462, 471, 475
Raghavacharya, D.B. Sir V., 567
Ragi, 47
Rahman, 293
Rajma, 20, 23, 47, 184, 286, 287, 290, 294,
551
Ramaiah, K., 582
Ramie, 582
Randhawa, M.S., 576, 583
Random
drift, 155, 160, 164, 277, 286
mating-see, under mating
mating population, 155, 156, 159, 340
Range, 102
Ranunculus , 42
Rapeseed, 409, 436, 516, 536
Raphanobrassica, 461, 462, 471, 472, 475,476,
490, 491, 494
Raphanus indicus-scc , radish
Raphanus sat iv us -sec, radish
Raspberry, 399, 403, 406, 409, 412
Rawlings, 115
Recessive, 74
Recipient, parent-scc, recurrent parent
Recombination, 81, 82, 139, 151, 174, 175,
257, 261, 281, 356, 372, 373, 433, 464,
483, 486,487,491,492
frequency, 81, 82, 282
somatic, 373
Recurrent parent, 62, 65, 237-253, 256, 257,
449, 450
Red clover, 459
Regeneration, 452, 498, 501, 502, 504, 505,
508
Regression coefficient partial, 107, 120, 121
Reproduction, 40-69
asexual, 40, 356-367, 463,
mode of, 40, 50, 51, 488, 554
parascxual, 371-373
rate, 160, 162, 163, 172. 178
sexual, 40 42, 46, 356, 358, 363, 371, 372
EJ *t»
3
Plant Breeding : Principles and Methods
vegetative 40, 41
Restorer gene-see, under gene line, 266, 237
Rhizobium, 4, 89, 593
Rhizopus , 370
R/hej, 489
Rice, 6, 13, 29. 42, 64, 65, 79, 140, 141 207
222, 228, 229, 499, 503, 504, 508, 593 ’
amhesis, 46
breeding, 13, 221, 248, 263, 281, 286, 287
disease, 370
disease resistance, 26, 31, 582 5S3 597
594,596 * * ’
emasculation, 146, 147
heterosis, 191
hybrid, 13, 328, 343
: : indica
insect pest, 398, 399, 403, 405, 408, 410
introduction, 24
japonica , 10
mutation, breeding, 424, 434, 436, 438
origin, 20
pollination, 47, 148
polyploidy, 453, 455
quality, 5, 488, 505, 596
seed production, 69, 534 537 539 54?
548,557 ’ M2 ’
varieties, 9, 10, 12, 24, 33, 35, 134 203
207, 222, 434, 436, 438, 490 504 583*
584, 592, 595, 596 1 ’ '
Rice bean, 10
Rirapau, 444, 476
Robertson, J.W., 527
Robinson, 118, 316
Rockefeller Foundation, 1 1 , 330 569 592 593
Roguing, 540, 554, 557-562 ’ ’ '
Romero, 291
Rose, 19, 41
Roy, 287
Rubber, 23, 25, 29, 36 49
Rubus, 357, 365, 366, 363, 471, 491
Russel, 121, 400, 527
RyC ’!f’^ 22 ’ 49 ’ 141 * 180,238,445,453
455, 459, 462, 471, 475, 487 488
Ryegrass, 55, 459, 489
Saccharutn , 32, 489
barberi, !0, 141, 250, 265, 581
noblisalion-sec, noblisation'
S?“’ !U7 ' 22 ' ,41 ' 250 ’ 265. 366,
robustmn, 365, 470
spontaneum, 11, 141, 365, 470 489
237 Safflower, 22, 23, 50, 51 , 60 275 ’ 51 1
Sagaret, 139 ’
Sage, 22
Salmon, 270, 271
207, Salvia , 34
>3 Sandal wood, 506
SBI, 11, 481, 580, 581
287 Schnell, 282
Sears, 466, 492
'92, Seoz/e, S p., 488, 500
Secale cereale -& Iso see, rye, 444 478 jq a
4gg ’
Seed, 526
Act, Indian, 528-530, 544
basis, 198, 554
i0 “I’ 198 ' 524 ’ 527 - 530, 532, 536, 537,
554, 556
certification, 208, 527, 530, 532-534 537
542, 545-554, 556, 557, 561 ’ ’
certify 518, 523, 524, 527, 529, 530-534,
536, 539, 547, 548, 554, 556
contaminated, 526
distribution, 537
*•' foundation, 216, 524, 527, 528 530 532
534-536, 539, 547, 548, 554, 555, 559, 560
genetic, purity, 526, 530-532 545 ’550
, 554, 555 * ’ ’ •
germination, 529, 532, 534, 552-554
hybrid, 31, 58, 329, 330
improved, 526-562
labelling, 529
law, 528, 550
moisture, 534, 541, 550, 554, 560 561
nucleus, 554, 561
physical purity, 534, 545
-plot technique, 562, 583
processing, 534-538, 540-546, 550
real value, 553, 554
registered, 527, 530, 554
substandard, 532
stock, 524
test, 527, 536, 537, 542, 545, 550-554, 556
test laboratory, 527, 528, 542, 550
treatment, 542, 543
utility percentage-see real value of seed
viability, 552-554
Seedproduction, 11, 458, 460, 536-540
'-Certified, 342, 557-562
?£ d L S2 - 58 ’ *>• 6i - 65 * 6S - MS,
P I47> 263 ■ 265-67, 330, 336-339, 342 343
493 ,540
organisation, 555-557
Index
635
Segmental allopolyploid, 444
Segregation, 74, 75, 80, 139, ,151, 174, 175,
356, 372, 373, 382
independent, of nonalleles, 75, 76, 80, 132
151
in autotetraploids, 455, 457
random chromatid, 456
random chromosome, 456
transgressive, 140, 151, 221, 233, 238, 239
250, 251, 257, 258, 274
Selection, 4, 7, 8, 24, 72, 78, 81, 88, 105, 109,
126-130, 134-136, 151, 155, 159-164, 167-
175, 194-200, 215-221, 227-233, 247, 258,
259, 269-296, 333, 340, 357, 388, 406, 434,
455, 457* 460, 464, 471, 480, 503
among crosses, 217, 218
artificial, 2, 15, 16, 17, 18, 126, 134, 224,
225, 227, 230, 233, 235, 328, 341, 351
bud, 358, 367
clonal, 356, 360-367
correlated response to, 169, 175, 291, 325
366 •
-differential, 134, 135, 162
ear-to-row, 300, 306-308, 320
full-sib, 309, 320, 321, 325, 353
gamete, 340
genetic advance, under, 101, 118, 134-135,
162, 218, 261, 272, 306, 321, 429
half-sib, 308, 321
inbred family-also see, S. and S, selection,
320, 321
index, 109, 110
individual plant-see, selection pureline
mass, 105, 133, 194-201, 203, 206-208,
210, 255, 260, 269, 286, 290, 291, 293-
295,300-306,309,310,329,406,530,554
modified ear-to-row, 309, 320, 324, 325
natural, 2, 16, 17, 18, 29, 126, 134, 174,
221 , 225, 227, 228, 233-235, 277, 286-290*
292-294, 341, 351, 482
permissible intensity, 162, 163
progeny, 300, 306, 308-311, 313, 320, 324
329
pureline, 3, 127, 138, 194, 195, 198, 199
201-209, 222, 255, 269, 301, 406
recurrent, 165, 258-260, 263, 300, 309-325
325
recurrent, GCA, 308, 311, 313, 319-325
351
recurrent, phenotypic, 301
recurrent, reciprocal, 308, 311, 316-325
351
recurrent, SCA, 308, 31 1 , 316-325
recurrent, simple, 308, 311-313
% progeny/family, 308, 320, 321
S 2 family/progeny, 308, 320, 321
stabilizing, 290
stratified, mass, 305, 325
visual, 215, 216, 219, 270, 271, 278, 280,
334
within crosses, 218-220
Self-fertility, 55, 58, 59
sem, 23, 47
Sen, Bosi, 585
Sesame-see, til
Sesamum indie um-stt, til
Set aria
italic , 481
viridis, 481
Shebeski, 270
Sheep, 416
Shull, 3, 178, 181, 183, 187, 329
Sick plots, 390, 391
Sickle cell anemia, 188, 190
Silk worni, 191
Simmonds, 394
Simmondsia-also see, jojoba, 584
Simplex, 455, 456
Sinpsis, 57
Singh, R.P., 277, 294
Singh, H.B., 33
Singh, H.S., 575
Singh, Sir I, 567 ■■■ '
Single seed descent (SSD), 231-233, 260, 269
272-281, 290-296
Sisal, 582
Skoog, 496
Smith, 109, 110, 291
Snape, 272, 281-282, 285
Solatium, 42, 53, 402, 459, 463
andigena, 365
curlilobum, 365
demissum, 365, 487
melongena-see, brinjal
nigrum, 444
tuberosum -see also potato, 479
' vemie, 406
Sorgtium-also see, Jowar, 552
bicolor-see, jowar
halepense , 539, 559^
Sour lime . : .
Soybean, 20, 33, 35, 47, 110, 259, 275, 278,
286, 287, 291, 292, 488, 537, 539
Spinacea oleracea- see, spinach
Spinach, 22, 49, 444, 446, 455
SSC, 526, 531, 538, 555-557
I
636
Plant Breeding : Principles and Method
SSCA, 526, 531, 536, 537, 545, 555-557
Sporogenesis, 43
mega, 43* 44, 45
micro, 43, 44
Stability, 120-123
Stadler, 133, 340, 416
Staminate, 42, 49
Standard deviation, 101, 134, 135, 162, 171
Standard error, 101
Stebbins, 481, 482
Sterility, male, 50, 52, 60-69, 147, 252, 260,
265, 336, 366, 433, 455, 483, 559-561 •
cytoplasmic, 50, 60, 62, 63, 64, 65, 69,
238, 259, 336, 337, 366, 493
cytoplasmic-genetic, 50, 60, 62, 64, 65, 66,
68, 265, 266, 330, 336, 337, 344, 482
genetic, 50, 60, 259, 236-338
maintenance, 60
Stevens, 288
Strawberry, 19, 49, 365, 366
Striga , 412, 594 «
Stuber, 109
Subculturing, 498
Substitution
base, 423
-line, 253, 447
alien, 470, 483-486
monosome, alien, 484, 486
Sugar beets, 35, 53, 346, 400, 401, 458, 552
breeding, 3, 19, 127, 202
disease, 388
disease resistance, 387, 388
heterosis, 191
hybrid, 337, 343
origin, 22
polyploidy, 455
Sugarcane, 17, 184, 357, 366, 476, 481, 483,
490, 494, 499, 502, 50 5, 567, 581
breeding, 10, 11, 46, 250, 365, 366, 367
disease, 360
disease resistance, 384, 385, 387
evolution, 19
germplasm, 26, 29, 31
insect pest, 398
propagation, 41, 356
varieties, 141, 367, 436, 438, 489, 490, 508,
581
Suneson, 225, 229, 287-289
Sunflower, 23, 29, 61, 64, 65, 68, 180, 183,
191, 343, 489
Surmtump, 47, 582 ' ^ ■' " ■
Suspensioo cultuce-see, under, culture -
Swaminathan, M.S„ 574
Sweet com, 539
Sweet potato, 17, 19, 23, 25, 31, 35, 46, 5'
145, 356, 357, 366, 457, 583, 584
Sweet William, 139, 475
Swine, 191
T
Taramira, 508
Taraxacum , 42, 463
Taro, 356
Taxus, 501
Tea, 26, 29
Tee , 272, 293-295
Temperature insensitive, 6
Teosinte , 489
Tester, 115, 116, 313-321, 329, 334
Tetraploid, 442, 444, 452, 453, 460, 471, 479
auto, 444, 455, 457, 461, 472
alio, 444, 461
Tetrasomic, 443, 445-447
Tetrazolium, 553
Theobromo , 29, 58
Theophrastus, 370
Threshold character, 78
Tigchelar, 277
Tilletia, 13
Til, 20, 22, 23, 25, 568
Tiniuthy grass, 180, 455
Tissue culture-see, under culture
Tobacco, 17, 29, 69, 207, 222, 480, 484, 487
493, 497, 504, 505, 508, 567, 583
aneuploidy, 445, 446,
breeding, 135, 259, 282
disease, 506
disease resistance, 506
emasculation, 145
evolution, 19, 466
germplasm, 26, 31
heterosis, 182, 205
insect resistance, 406
introduction, 25, 33
origin, 23
quality, 488
seed production, 267
varieties, 35, 207, 209, 283
weed, parasitic, 412
yield, 489
Tomato, 488, 489, 505
aneuploidy, 444, *446, 450, 452
disease, resistance, 487
emasculation, 145
evloution, 18
Index
fgpjfp
haploidy, 454
heterosis, 182, 184, 191, 265
hybrid, 183, 328, 343
insect resistance, 403
introduction, 33
origin, 23
pollination, 47
quality, 488
see production, 61, 266, 359, 534
varieties, 33, 35, 222
weed, parasitic, 412
Toria, 455, 468
Totipotency, 496, 497
Transformation, 507
Transgressive
breeding, 140, 144, 213
segregation-see, under segregation
Transition, 423, 424
Translocation, 415, 433, 445, 447 481 486
487
Transposon, 505, 507, 508
Transversion, 423
Trial, 510
adoptive, 510, 516, 517, 520, 522
agronomic, 515-519, 522, 523
coordinated, 196-198, 206, 516
initial evaluation, 198, 515, 516, 517, 522
minikit, 510, 516, 517, 518, 524, 537
model agronomic, 515-517, 522
multilocation, 510, 511
national 510, 516, 517, 522
national demonstration, 344, 537
preliminaiy, 204, 212, 215, 216, 510
state varietal, 223, 524
station; 510, 511
uniformity regional, 198, 515, 516 517
522
Tpallel analysis, 114, 115
Trifolium, 53, 58, 60
pratense, 60
resupinalum-sez , Persian clover
Triple fusion, 44
Triple test cross analysis, 119, 120 274
Triplex 455, 456
Triploid, auto-19, 443-445, 452, 455, 458 459
460 V ’
Tripsacum, 19, 477, 479, 488
Trisomic, 443-447, 451, 452, 455
analysis, 445, 451, 452
double* 443, 455
Tritkale, 19, 141, 270, .271, 444, 461, 462,
471, 472, 476, 490, 491, 494, 500, 503,
Triticum, 22, 141, 444, 462, 468, 480 500
552
aestivum - also see, wheat; 238, 239, 466,
470, 478, 483, 487, 489, 500, 501
compaction , 22
dicoccoides
dicoccwn , 22, 23
durum , 22, 23, 64
monococum , 238, 466
trimopheevii , 64, 68, 82, 238, 239 483
489
turgidum, 23, 480
Triticum- Aeg Hops hybrids, 491, 494
T riticum-Agropyron hybrids, 494
Tulipa, 463 «
Turmeric, 22, 356, 502, 585
Turnip, 20, 22, 35, 401, 455, 459, 460
UNDP, 591
Univalent shift, 452
Universities, ariculture, 11
Urid, (Urd), 47, 107, 108, 109, 203
USAID, 575
UV rays, 419, 421, 552
Van der Plank, 378, 386-
Vanilla, 489
Van Mons, 127 ; ’ ; \ i
Variability, 12, 16, 29, 30, 86, 87,' 100-104
164, 166, 168, 169, 171, 255, 282 313
314, 360
creation of, 8, 138, 139, 255, 260, 360
371, 372, 472 ’
erosion of, 277
free 174, 175
genetic, 105, 134, 169, 174,’ 198,201, 348
352, 406, 472
natural, 8, 138
origin of, 18
potential, 174, 175:
reduction in' 12, 17
Variance, 89, 90-97, 101, 105, 106, 111, 117-
123, 162, 165, 167, 168, 171, 179, 184
273-275, 281, 290 ' '
additive, 91, 92, 95, 96, 101, 113, 118, 172
273, 292, 302, 321, 324, 325
analysis of, 88-91, 95, 96, 1 19, 121, 122
dominance, 91, 92, 95, 96, 101, 111, 113,
118, 136
638
Plant Breeding : Principles and Methods
environmental, 90, 92, 95, 96, 100, ill,
274
epistatic, 91, 92, 96, 101, 113, 115, 116,
118,136,272
error, 88-90
genetic-see, genotypic, variance
genotypic, 89, 90, 91, 92, 95, 96, 97, 100,
101, 134-136, 172, 218, 324
interaction-see, epistasis variance
nonadditive, 101
phenotypic, 89, 90, 95, 96, 100, 101, 134-
136
Variation, 3, 27, 72, 126, 128, 130, 194, 292,
.357
creation of, 7, 8, 46, 126, 285
gametoclonal, 504
genetic, 194, 207, 285, 289, 357-359, 371,
398400, 429, 504, 505
Mendeli&n, 18
somaclonal, 8, 504*506, 508, 581
Variety .
composite, 11, 328. 331, J45, 350, 351,
353, 326
desi-see, land, varieties
dwarf, 2, 9, 10, 12, 24, 134, 595
high-yielding, 1
hybrid, 3, 11, 12, 30, 50, 52, 140, 183,
191 , 255, 263-267, 300, 320, 328-345, 347,
348, 371, 372, 526
identification of, 9, 510, 516, 517, 519, 520,
522,523
improved, 1, 9, 11, 12, 492, 526, 532
introduced, 203
' land, 11, 12, 29-33, 195, 198, 199, 202,
203, 492
local-see, land, variety
multiline, 252, 255-257, 293, 410
notification, 510, 520, 522, 523, 529, 532
open-pollinated, 10, 30, 32, 51, 169, 171,
172, 183, 313, 314, 316, 325, 329, 330,
331
pureline, 12, 17, 29, 30, 195, 198, 200-
203, 206, 234, 252, 255, 256, 259, 265,
597
release of, 9, 198, 510-524, 588
stability of, 51, 52
synthetic, 51,52, 191,300, 311,313,314,
316, 328, 329, 331, 345-353, 526
Vavilov, NX, 20, 23, 33
Vector, insect-see, under insect
veniuria inequaelis , 375
Venkatraman, T.S., 11
V enir icillium, 582
Vertifolia effect, 386
Vetch, 22
Vigna, 489
anguiculata-see, cowpea
radiata- see, also mung, 480
umbellata, 480
Vilmorin, 3, 127, 202
isolation, principle-see, progeny test
Virulence, 374, 375, 379-386
Virulent, 374, 375, 379-386
W
Walnut, 25, 49
WARDA, 592, 593
Warner, 118
Water hyacinth, 36
Watermelon, 19, 35, 383, 387, 455, 458, 460,
534
Weber, 287, 291
Weed, noxious, 534, 536, 539, 540, 547-551,
550-562
Weinberg, 155
Whaley, 189
Wheat, bread/hexaploid, 6, 13, 17, 20, 40, 45,
65, 68, 69, 76, 83, 84, 133, 140, 141, 150,
207, 222, 228, 251, 447, 464, 471, 475,
477, 483, 484, 487-490, 492, 503, 504, 508,
580,592,594
aneuploidy, 445-447
breeding, 13, 127, 202; 218, 221, 237, 270,
277, 281, 283, 286, 287, 290, 291, 293
disease, 13, 36, 373, 378, 380, 391
disease resistance 82, 248, 257, 369, 370,
380-384, 386-388, 392, 393, 487, 493
dormancy, 17
erriasculation, 145, 147
evolution, 19, 466, 467
fertility;' male, 64
germplasm, 31, 33
heterosis, 191, 265
insect pest, 397, 401, 403-405, 409, 411,
412
introduction, 24, 388
mutation breeding, 424, 434, 436
origin, 22
pollination, 47, 148
pollyploidy, 453 ,
quality, 5, 505/518, 519, 596
seed, 527, 534,-537, 539, 542/551, 555,
557, 558
trial, 511, 515, 516, 517, 515, 519, 522
varieties, 9, 12, 13, 24,. 33. 34, 203, 207,
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